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Identification Marks on Tools (Excluding Sockets)

After reviewing the registered trademarks in the previous section, let's look now at the various informal marks and brands used by Duro and Indestro. Where possible, the figures will show combinations of an informal mark with a registered trademark, in order to establish a firm connection with the companies' products.

Chromium Vanadium

As chrome alloy tools became more in demand, Duro and Indestro started using the mark "Chromium Vanadium" to indicate these high-end (and more expensive) tools. Although many other manufacturers also made chrome alloy tools, these were usually marked using the abbreviated "Chrome Vanadium" phrase, so the use of the full element names can help identify particular Duro and Indestro pieces.

The date for the first use of the Chromium Vanadium mark is not known, but we do have direct evidence that it was in use by 1931. By fortunate circumstance, we were able to acquire an early Indestro "Chromium Vanadium" socket set with a dated sales receipt from 1931. The set consists of ten 12-point sockets, a speeder, T-slider, ratchet adapter, extensions, and a universal, with most of the pieces being marked "Chromium Vanadium". The speeder is also marked "Indestro Chicago", and the metal box has an Indestro decal on the top.

The set was purchased from a Western Auto store, and a check of their 1932 catalog shows that a "Chromium Vanadium" socket set was available, with a description exactly matching the set shown below. The Western Auto Supply connection provides an explanation for Indestro's use of this mark -- Western Auto had begun using "Chromium Vanadium" as a brand in its catalogs around 1931, and as a supplier to Western Auto, Indestro probably found it easier to just mark all of their alloy tools with "Chromium Vanadium".

Indestro 1/2-Drive Chromium Vanadium Socket Set

Indestro 1/2-Drive Chromium Vanadium Socket Set]
Fig. 4. Indestro 1/2-Drive Chromium Vanadium Socket Set, 1931.

Fig. 4 shows the Indestro 1/2-drive Chromium Vanadium socket set from 1931. The ten sockets are marked "Chromium Vanadium" except for one, and all of the drive tools are similarly marked, except for the unmarked universal and double-male extension.

The sockets in the set will be examined in greater detail in a later section, when we look at 12-Point Sockets.

Indestro 1/2-Drive Chromium Vanadium Speeder]
Fig. 5. Indestro 1/2-Drive Chromium Vanadium Speeder, with Insets for Detail, 1931.
Indestro 1/2-Drive Chromium Vanadium Sliding Tee Handle]
Fig. 6. Indestro 1/2-Drive Chromium Vanadium Sliding Tee Handle, 1931.

Indestro "Chromium Vanadium" 1/2-Drive Ratchet Adapter

Indestro 1/2-Drive Chromium Vanadium Ratchet Adapter]
Fig. 7. Indestro 1/2-Drive Chromium Vanadium Ratchet Adapter, 1931.

Fig. 7 shows the ratchet adapter from the set, marked "Chromium Vanadium" and "1-2" with stars on either side.

The overall length (extended) is 3.2 inches.

The 1932 Western Auto catalog called this tool simply a ratchet rather than a ratchet adapter. By 1933 the Western Auto socket sets included a true ratchet instead of the adapter, a model based on Duro's patent #1,902,878.

Indestro 1/4-Drive "Chromium-Vanadium" Socket Set

[Indestro 1/4-Drive Chromium Vanadium Socket Set]
Fig. 8. Indestro 1/4-Drive "Chromium-Vanadium" Socket Set, ca. 1935-1936.

Fig. 8 shows another Indestro socket set, this time a 1/4-drive set with "Chromium-Vanadium" prominently featured in the logo decal. In this set the individual tools are not marked except for sizes on the sockets. (This set will be mentioned again in the section concerning socket identification.)

Indestro Polygon

Indestro made use of an "Indestro Polygon" brand around the same time that the "Chromium Vanadium" mark was in use. Currently the known examples are all offset box-end wrenches with 12-point openings, but it may have been used for other tools as well.

Although the dates are uncertain, the Indestro Polygon wrenches are believed to have been made in the late 1920s or early 1930s.

Indestro Polygon Offset Box-End Wrench

This next figure shows an example of the Indestro Polygon brand.

[Indestro Polygon 5/8x11/16 Offset Box-End Wrench]
Fig. 9. Indestro Polygon 5/8x11/16 Offset Box-End Wrench, with Insets for Side View and Reverse Detail, ca. Early 1930s.

Fig. 9 shows an Indestro Polygon 5/8x11/16 offset box wrench, stamped "Chromium-Vanadium" on the reverse.

The overall length is 9.8 inches, and the finish is plain steel.

The upper inset shows a side view of the wrench to illustrate the construction details. The shank is roughly oval in cross-section, with a smoothly rounded top and bottom and clipped sides.

A particular detail to note is that the offset shank joins the box end over the full width of the box, without a lip above and below the join.

Later construction for Duro and Indestro offset box wrenches generally had the shank joining near the middle of the box end, with a distinct lip above and below. See for example the Indestro Super 731A Wrench shown later in this page.

Indestro Polygon Chromium Vanadium Wrenches

[Indestro Polygon Wrenches with Chromium Vanadium]
Fig. 10. Indestro Polygon Wrenches with "Chromium Vanadium" Mark.

Fig. 10 illustrates the connection between the Polygon and "Chromium-Vanadium" marks, showing the reverse side of three Indestro Polygon wrenches.


Somewhat after the use of the "Chromium Vanadium" mark, Indestro started using the phrase "Super-Quality" as a brand for its Chrome alloy tools. This use of "Super-Quality" is the obvious forerunner for the eventual Indestro "Super" line of chrome-alloy tools.

[Indestro Super-Quality Socket Set]
Fig. 11. Indestro Socket Set of Chrome Vanadium Steel with "Super-Quality" Logo.

Fig. 11 at the left shows one example of this, a 1/4-drive socket set with an Indestro logo showing "Super-Quality" and "Chrome Vanadium Steel". The tools in this set are marked "Chromium Vanadium", and some have part numbers that in later catalogs would be identified as part of the Indestro "Super" line.

Although the time of the first use of "Super-Quality" is unknown, the mark was definitely in use by 1938, as the Indestro catalog for that years shows a number of examples. In this early usage the "Super-Quality" mark apparently referred not only to the alloy steel, but to a premium chrome-plated finish as well.

A 1941 Indestro catalog shows extensive use of the "Super-Quality" mark for the chrome alloy tools, both on the tool illustrations and on advertising displays. Interestingly, the sockets shown are referred to as "Super-Quality" in the text but are marked "Chromium Vanadium" in the illustrations, suggesting that the change may have been phased in over a period of time.

[Duro/Indestro Wrenches with Super-Quality and X-Circle Marks]
Fig. 12. Duro-Chrome and Indestro Wrenches with Super-Quality and X-Circle Marks.

In Fig. 12 we see several more examples of the use of the "Super-Quality" mark, this time on the tools themselves, along with other identifying marks.

The wrench at the top is marked Indestro and has the part number 714, which later catalogs show as part of the Indestro Super line. The two middle wrenches are marked with "Super-Quality" and the X-Circle mark, and in addition show the streamlined design in the polished raised panel area.

The bottom wrench is marked Duro-Chrome 2013 and shows the X-Circle mark as well. The raised panel streamlined design is comparable to that in the middle two wrenches, showing the commonality of design among all four tools.

Indestro 763 9/16x5/8 S-Shaped Box-End Wrench

[Indestro 763 9/16x5/8 S-Shaped Box_End Wrench]
Fig. 13. Indestro 763 9/16x5/8 S-Shaped Box-End Wrenches, with Inset for Reverse Detail.

Fig. 13 shows another example of the "Super-Quality" mark, an Indestro 763 9/16x5/8 S-shaped box-end wrench marked "U.S.A." and "Super-Quality". The inset shows the reverse with "Chrome Vanadium Steel" in a particular slanted font, which will be discussed in a later section.

The overall length is 8.0 inches, and the finish is plated chrome with polished box ends.

Indestro Super

We've mentioned the Indestro Super brand several times as the successor to the Super-Quality mark, so it's time to show some examples. The transition date from Super-Quality to Indestro Super is a bit uncertain, as we currently don't have sufficient catalog coverage or other information. However, the first example below at least suggests a likely time frame.

Indestro Super 2 Tappet Wrench

[Indestro Super Model 2 1/2x9/16 Tappet Wrench]
Fig. 14. Indestro Super Model 2 1/2x9/16 Tappet Wrenches, with Inset for Reverse Detail, ca. 1944-1945.

Fig. 14 shows an Indestro model 2 1/2x9/16 tappet wrench, stamped "Indestro Super" on the face with "Forged in U.S.A." on the reverse. The overall length is 8.0 inches.

The finish is plain steel with traces of the original cadmium plating, and this provides a helpful clue to the manufacturing date.

Indestro and Duro-Chrome wrenches were generally given chrome plated finishes after the early 1930s, except for the period of wartime shortages in 1944-1945. Tools made at that time generally substituted cadmium (or plain) finishes for chrome, so it's likely that this wrench was made in 1944-1945.

This tool is probably our earliest example of the Indestro Super marking, and indicates that the transition likely occurred around that time.

Indestro Super 731A 9/16x5/8 Offset Box-End Wrench

[Indestro Super 731A 9/16x5/8 Offset Box-End Wrench]
Fig. 15. Indestro Super 731A 9/16x5/8 Offset Box-End Wrench, with Insets for Side View and Reverse Detail.

Our next example is shown in Fig. 15, an Indestro Super 731A 9/16x5/8 offset box-end wrench. The hexagonal shank has raised panels in the streamlined design, with "Indestro Super" on the front and "Forged in U.S.A." on the reverse.

The overall length is 9.4 inches, and the finish is chrome plating with polished ends.


The next figure shows a recently acquired tool that proved to be the "missing link" to a brand name used by Duro in the late 1920s or early 1930s. The front side of the wrench is clearly marked as Duro-Chrome model number 268, which is listed as a Chrome Vanadium brake wrench (Ford-specific) in catalog 38M and others. But on the back side (see inset), it's also marked as "Handy-Hex".

Prior to the discovery of this wrench, the "Handy-Hex" mark had been the source of a long-standing mystery, as a number of wrenches of this brand had been acquired, but no references had been found to identify the maker.

Duro-Chrome 268 "Handy-Hex" 7/16x1/2 8-Point Brake Wrench

[Duro-Chrome 268 7/16x1/2 8-Point Brake Wrench]
Fig. 16. Duro-Chrome 268 7/16x1/2 8-Point Brake Wrench, with Inset Showing "Handy-Hex" Brand.

Fig. 16 shows a Duro-Chrome 268 7/16x1/2 8-point brake wrench, marked "Chrome Vanadium Steel" with "Ford Brakes" and "Handy-Hex" on the reverse.

The overall length is 11.0 inches.

After this wrench turned up, the markings on some older Handy-Hex pieces were reexamined to look for previously missed clues. Two of the available examples were deep-offset wrenches marked "Chrome Vanadium Steel", with model numbers 02055 (13/16 x 7/8) and 02056 (15/16 x 1). (One of these Handy-Hex wrenches is shown in the next figure below.)

When the Duro-Chrome catalog 38M was checked for this style of deep offset wrench, it listed model number 2055 with sizes 13/16x7/8, model 2056 with sizes 15/16x1, and an overlooked caption jumped out: "Handy-Twin-Hex"!

Handy-Hex 02055 13/16x7/8 Offset Box-End Wrench

[Handy-Hex 02055 Box-End Wrench]
Fig. 17. Handy-Hex 02055 13/16x7/8 Wrench, with Inset for Reverse Detail.

Fig. 17 shows a Handy-Hex 02055 13/16x7/8 offset box wrench, marked "Made in U.S.A." with "Chrome Vanadium Steel" on the reverse.

The overall length is 12.6 inches, and the finish is a thin plating with a yellowish tint.

Select Steel

Duro and Indestro began using the phrase "Select Steel" in several variations as an informal trademark fairly early on, probably as soon as they needed to make a distinction between the more expensive Chrome Alloy steel and the less expensive "Select" alloy. Many of their tools are marked with the phrases "Forged Select Steel" or "Drop Forged Select Steel" (or other variations), and the catalogs have a section for "Select Steel" economy tools as well as the high-end "Chrome Alloy" tools. (As previously noted in the section on trademarks, Indestro did later register "Select Steel Tools" as a formal trademark.)

[Indestro Chicago Wrenches]
Fig. 18. Indestro Chicago Open-End Wrenches Showing "Drop Forged Select Steel".

Fig. 18 at the left shows the use of "Drop Forged Select Steel" on the reverse side of the Indestro Chicago wrenches, previously shown in the discussion of the Streamlined Design Trademark.

With the history of consistent use of "Select Steel" as a brand, this should serve as a clear identification for Indestro tools, especially when present with other distinctive marks.

The X-Circle Mark

We've seen several examples of the X-Circle mark previously (e.g. in Fig. 12 above) and will now examine this marking more closely.

[Duro-Chrome 2015A Box-End Wrench]
Fig. 19. Duro-Chrome 2015A 13/16x7/8 Box-End Wrench, with Insets for Reverse Side and X-Circle Logo.

Fig. 19 shows a Duro-Chrome 2015A 13/16x7/8 box-end wrench, stamped with a somewhat larger than normal X-Circle along with the usual markings.

Although no documentation for the origin of this mark has been found, a close examination of the logo in Fig. 19 suggests its likely meaning. Note that the sides of the "X" are actually curved, so that the mark resembles the letters "DC" butted together, with the circle overstriking the left edge of the "D". Thus the mark is likely a stylized "DC" representing Duro-Chrome, similar to the NB-Circle logo used by New Britain with its "None Better" trademark.

We'll refer to this mark as the X-Circle logo in the text.

As the X-Circle marking is almost always found on tools with plain finishes or other signs of wartime production, this mark is is presumed to be Duro's identification for tools produced under wartime contracts. Further examples of the X-Circle mark can be found in the section on Duro/Indestro Wartime Production.

Duro-Chrome 2081 Half-Moon Wrench

[Duro-Chrome 2081 9/16x5/8 Half-Moon Box Wrench]
Fig. 20. Duro-Chrome 2081 9/16x5/8 Half-Moon Box Wrench, with Inset for Reverse.

Fig. 20 shows another example of the X-Circle mark, this time on a Duro-Chrome 2081 9/16x5/8 half-moon wrench, marked "Forged in U.S.A." with "Manifold & Starter" and an X-Circle on the reverse.

The overall length is 7.2 inches, and the finish is plain steel.

This style of wrench is referred to as an "elbow wrench" in the Duro catalogs. The nearly identical (except for markings) Indestro 769 model is shown in Fig. 21 below.

Based on the observed patterns of usage, the X-Circle mark was used to indicate production under Duro's government contracts during the 1942-1945 wartime years. The main evidence for this interpretation is that tools with the X-Circle mark are generally observed with plain or cadmium finishes, or with other signs of wartime production, such as roughly milled faces on open-end wrenches.

Some tools were marked with X-Circle as the only indication of the company, but in other cases the X-Circle mark appears along with a Duro-Chrome or Super-Quality brand. These latter examples can be interpreted as production originally intended for civilian markets, but diverted to a government contract in order to meet production quotas.

Other Marks and Characteristics

In addition to the various marks described above, there are several other indications that may help to identify Duro/Indestro tools, at least if present in conjunction with other distinctive marks.

Slanted Font with "Chrome Vanadium Steel"

One important identifier for Indestro tools is the use of the phrase "Chrome Vanadium Steel" (or "Chromium Vanadium Steel") in a distinctive slanted font. This next figure shows an example.

[Indestro 769 9/16x5/8 Half-Moon Wrench]
Fig. 21. Indestro 769 9/16x5/8 Half-Moon Wrench, with Inset Showing Slanted Font.

Fig. 21 shows an Indestro 769 9/16x5/8 half-moon wrench, marked "U.S.A." with "Super-Quality" on the front side. The inset shows the reverse side with an example of "Chrome Vanadium Steel" in the slanted font.

Although this slanted font could have been used by other manufacturers, currently the only examples known are on tools marked as or of apparent Indestro production. (Tools marked with Duro brands or model numbers have not been observed to use the slanted font.)

Another example of the slanted font appears can be seen on the Indestro 763 S-Shaped Wrench.

Curved Arc "Forged in U.S.A." Mark

Another marking style noted only on Duro-Chrome and Indestro tools is the use of the phrase "Forged in U.S.A." curved along a circular arc, typically stamped on a wrench head.

[Duro Wrenches with Forged in U.S.A. Arc]
Fig. 22. Circular Arc "Forged in U.S.A." Mark on Several Duro-Chrome Wrenches.

Fig. 22 shows several examples of the "Forged in U.S.A." curved-arc marking.

Examples of this marking have been found on tools marked "Duro-Chrome", "Super-Quality" and "Indestro Super", or otherwise marked to indicate an alloy steel tool, but not on economy-grade Select Steel tools.

Patent Numbers and Dates

Duro Metal Products was fairly prolific in its patent activities, as we saw in the Patent Table shown in the previous section. The tools produced under these patents were often marked with the patent number or date, and such patent markings may then serve as an important identification mark.

The figures below will show some examples of Duro and Indestro tools with patent markings.

Duro 672 1/2-Drive Ratchet Handles, Patent #1,798,481

In some cases the patent number may be the only identifying mark on a tool, perhaps to allow the tool to sell with various private brandings, or with no other identification at all.

[Duro 672 1/2-Drive Ratchet Handles]
Fig. 23. Duro 672 1/2-Drive Ratchet Handles Showing Patent Status.

An example of this is shown in Fig. 23, where we see three examples of the Duro model 672, an early ratchet handle. The ratchet at the top is marked "Duro Metal Products Co. 672" and "Patent Pending", and the one in the middle is similarly marked, except that it shows the actual patent #1,798,481.

The ratchet on the bottom is identical to the other two and shows the same patent number, but is marked with just "Chicago, U.S.A.", without a maker or model number.

The explanation is simple: the bottom ratchet came from an Indestro-branded socket set, and in order to maintain the separation of the brand names, Indestro simply omitted the Duro branding and model number.

The patent information on these ratchet handles provides an opportunity to estimate the production date of a tool. The first ratchet handle, being marked "Patent Pending", would have been made during the interval between the filing of the patent application and the subsequent issue. The other ratchets are marked with the actual patent number, so by examining the document for patent #1,798,481, we can determine that the first ratchet was likely made between 1928 and 1931.

Indestro Socket Set, Patent #1,788,535

[Indestro 1351 Socket Set]
Fig. 24. An Indestro No. 1351 Socket Set, with Inset Showing Patent 1,788,535.

Fig. 24 below shows an example of a socket set identified only by a patent number, in this case the Duro Metal patent #1,788,535. After reviewing an Indestro catalog, the socket set matches the description and appearance of model No. 1351, but none of the tools are marked with a company name or part number. However, the bottom of the case is stamped with the patent number, as shown in the inset to Fig 11B. (The vintage price sticker is interesting as well.)

Duro 1/2-Hex Drive L-T Convertible Handle, Patent #1,744,413

[Duro 1/2-Hex Drive L-T Convertible Handle]
Fig. 25. Duro 1/2-Hex Drive L-T Convertible Handle, ca. 1930s.

Fig. 25 shows a Duro 1/2-hex drive convertible L-T handle, a tool frequently included in Duro and Indestro economy-line socket sets. The handle is marked only with "Made in U.S.A" and "Pat. No. 1,744,413"; the omission of the company name indicates that the tool was probably intended for contract production.

The finish is a heavy cadmium plating.

This handle was acquired as part of a socket set in a box with a sliding lid.

The patent #1,744,413 was issued in 1930 to E.H. Peterson, one of Duro's most active inventors. In operation, the tool converts to a Tee-handle by sliding the Ell-handle bar out of the sheath, then inserting the short arm of the Ell into the side of the sheath. The arm locks in place against an indentation in the wall of the sheath.

The Duro L-T handle was also made in a 1/2-drive version as the Duro No. 660 1/2-Drive L-T Handle. The No. 660 L-T handle was used in a number of early Duro/Indestro socket sets, including sets sold by Sears Roebuck under the Durobilt brand.

The Duro L-T tool is similar in operation to the Hinsdale Convertible T-L Handle; however, the Hinsdale tool is permanently assembled.

Duro-Chrome 678D 1/2-Drive Ratchet, Patent #1,902,878

One of the more notable Duro inventions described a novel way for constructing ratchets, and was issued as patent #1,902,878 in 1933. Instead of the conventional ratchet assembly held together with screws or rivets, the Duro design secured the parts with a pressed flange integral to the forging. This allowed Duro to produce a high-quality forged ratchet at a reasonably low cost, and the resulting products were sold widely into high-volume markets.

Probably the first market for these ratchets, and perhaps the impetus for their development, was as part of the Chrome-Vanadium Socket Wrench Set offered by Western Auto in their 1933 catalog. This set sold for a mere $5.68 and included the ratchet, a speeder, a T-slider, two extensions, a universal, and ten 12-point sockets, truly a stunning value for a set of alloy steel tools. (This set was very similar to the Indestro Chromium Vanadium Set shown earlier, with the ratchet adapter replaced by the pressed-flange ratchet.)

[Duro-Chrome 678D 1/2-Drive Ratchet]
Fig. 26. Duro-Chrome 678D 1/2-Drive Ratchet, with Inset for Reverse, ca. 1932-1936.

Fig. 26 shows our first example of the pressed-flange design, a 1/2-drive Duro-Chrome model 678D reversible ratchet, marked with a bold "Duro-Chrome" on a raised panel in the center of the forged handle. The reverse shows a similar panel with "Made in U.S.A." and "Pat. No. 1902878". The overall length is 10.4 inches.

The finish is plain steel with a dark oxide coating.

This ratchet has a fairly coarse 18-tooth gear, but later versions of this model had a 36-tooth mechanism for a much finer action. The combination of the coarse action and plain finish suggests that this may be a very early model, possibly dating to the early 1930s.

One interesting feature is the provision of both male and female drive, as can be seen in the photograph. Male drive ratchets were becoming more popular in the 1930s and were gradually replacing the older female models. By providing both drive genders, Duro allowed the ratchet to work with older sets having double-male extensions and drive plugs.

The 1938 Duro-Chrome catalog also offered a similar model 677D non-reversible ratchet, fitted with a female-only drive gear. The pressed-flange ratchet design remained in production at least until the early 1960s, with catalog 37-M offering a model 676 female drive ratchet with a 36-tooth action.

Duro-Chrome 4487 1/4-Drive Ratchet, Patent #1,902,878

[Duro-Chrome 4487 1/4-Drive Ratchet]
Fig. 27. Duro-Chrome 4487 1/4-Drive Ratchet, with Inset for Reverse.

The pressed-flange construction was also used for other drive sizes, as can be seen in Fig. 27, a 1/4-drive Duro-Chrome model 4487 ratchet. The handle has a forged "Duro-Chrome" on a raised panel, and a similar panel is marked "Made in U.S.A." and "Pat. No. 1902878" on the reverse. The overall length is 6.0 inches, and the finish is cadmium plating.

The ratchet mechanism has a 20-tooth gear and is fitted with a reversing lever, as with the previous example. This ratchet also offers both male and female drive.

Duro-Chrome 2114 Battery Pliers, Patent #D119,441

The next figure shows a pair of battery pliers with unusual box-end wrench handles, marked with a design patent.

[Duro-Chrome 2114 Battery Pliers]
Fig. 28. Duro-Chrome 2114 Battery Pliers.

In Fig. 28 we see the Duro-Chrome 2114 battery pliers, marked "Forged in U.S.A." with "Des. 119,441" for the design patent.

The design patent #D119,441 was issued on Mar. 12, 1940 to W.A. Sandy and assigned to Duro Metal Products.

That completes the section on the informal brands and marks useful for tool identification. In the next section, we'll take a look at Duro and Indestro sockets.

Socket Identification

The early sockets produced by Duro and Indestro are frequently difficult to identify, as they were typically marked only with the size and possibly an obscure symbol. In some cases, the only guide to the maker is the overall design and construction of the socket, and this too may be ambiguous; other manufacturers' early sockets were made in a similar manner.

In this section we'll look at a number of examples of sockets and socket sets from Duro and Indestro, and develop guidelines and characteristics for identifying the sockets.

Early Hex-Drive Sockets

Sets of hex-drive sockets in metal boxes were offered by a number of manufacturers, at least as early as 1915. It's likely that Duro and Indestro offered these from the early 1920s or before.

Indestro No. 220 1/2-Hex Drive Socket Set

Early Indestro No. 220 1/2-Hex Drive Socket Set]
Fig. 29. Early Indestro No. 220 1/2-Hex Drive Socket Set.

Fig. 29 shows an early Indestro No. 220 1/2-hex drive socket set, complete with a steel container with an embossed lid. The fractional sizes are written in the style "5-8" rather than expected "5/8" form; this appears frequently on early sockets, though some socket sets have the syles in mixed format.

Note that the sockets are marked with five-pointed stars on either side of the size markings; this pattern appears on several examples of Indestro sockets, suggesting that this was intended as a specific mark for Indestro. (As a counterargument though, early sockets from Bog Mfg. of Chicago also show similar markings with stars on either side of the size.)

Sockets from Duro LTX316 1/2-Hex Drive Socket Set

[Duro 1/2-Hex Drive Sockets]
Fig. 30. Duro 1/2-Hex Drive Sockets, ca. 1929-1930.

Fig. 30 shows two of the 1/2-hex drive hex sockets from the Duro LTX316 Socket Set, with sizes 3/8 and 7/16. (Note that the opening shown in the photograph is the 1/2-hex drive end.)

Most of the sockets in the LTX316 set are marked only with the fractional size, but several of them also have a distinctive trapezoid symbol resembling a stylized "D". This trapezoid symbol was typically stamped on each side of the size and has been identified by a catalog illustration as a mark for Duro production. This symbol will be referred to as the D-Trapezoid logo in the text, and additional examples will be shown later with the Duro 1/2-Drive Sockets.

Early 1/2-Square-Drive Sockets

The 1/2 square drive size had emerged as a standard well before 1920 and was the preferred size for more demanding socket wrench applications. The early makers of these tools included Blackhawk, Snap-On, and Walden, and it's likely that Duro and Indestro had joined the group by the early to mid 1920s.

In the article we'll refer to 1/2-square drive as simply 1/2-drive when the meaning is clear from the context.

Indestro 1/2-Drive Hex Sockets

[Early Indestro 1/2-Drive Hex Sockets]
Fig. 31. Early Indestro 1/2-Drive Hex Sockets.

Fig. 31 shows a group of three early 1/2-drive hex sockets from an Indestro socket set. Note that the markings include five-pointed stars on either side of the size, a feature noted in the earlier No. 220 Socket Set as well as on the Indestro Socket Wrench shown elsewhere in this article. The production date of this set is unknown, but likely dates from the mid to late 1920s.

The construction of these sockets is similar to those in the No. 220 set. The sockets were first machined, then cold-broached for the six-point hex opening, and finally hardened; they also appear to be finished with a thin bright plating.

[Broaching Detail for Early 1/2-Drive Indestro Sockets]
Fig. 32. Broaching Detail for Early 1/2-Drive Indestro Sockets.

Fig. 32 shows a closeup of the socket construction. If you look carefully at the upper righthand socket, you can see a small burr left by the broaching, showing that the broaching came after the recess was machined. The centers of the flats also show tooling marks where the socket was drilled out prior to broaching.

Early Duro 1/2-Drive Sockets from No. 825 Socket Set

[Early Duro 1/2-Drive Sockets from No. 825 Socket Set]
Fig. 33. Early Duro 1/2-Drive Sockets from No. 825 Socket Set, ca. 1928-1931.

Fig. 33 shows a group of three 1/2-drive Duro sockets from an early Duro No. 825 Socket Set believed to be representative of early Duro production. These are from a socket set that included a Duro Metal 672 ratchet marked "Patent Pending", apparently original with the set and implying production around 1928-1931.

The sockets in the photograph are, from the left, a 25/32 hex socket viewed from the service end, a 5/8 square socket viewed from the service end, and a 1/2 hex socket viewed from the drive end.

The sockets in this set are very similar in construction to those in the early Indestro set discussed above, but are marked with an odd trapezoidal symbol (resembling a stylized "D") on either side of the size, where the Indestro sockets used a five-pointed star.

This "D"-like trapezoid symbol had been assumed to be an identification mark for Duro, an assumption that was confirmed by an illustration in a 1939 Duro catalog. The catalog shows an illustration on page 40 of a "Special Carbon Manganese Steel Socket", with the two trapezoid symbols clearly visible on either side of the size marking. We'll refer to this symbol in the text as the D-Trapezoid logo for ease of reference.

Apart from the markings, another difference from the (likely earlier) Indestro sockets is that the knurling on the Duro sockets is now a very fine cross-hatch, giving them a more refined appearance. The coarse knurling on early sockets was probably intended as a functional feature, to allow the socket to be turned easily by hand, but later evolved to a more decorative role.

Although these sockets are marked only with the D-Trapezoid and size, the 1939 catalog shows the model numbers that were used for the series. The numbers have the form "99xx" where "xx" is the size in 32nds, so for example model 9924 indicates a 3/4 size. The square and double-square broachings are identified with an appended "S", e.g. model 9920S for the 5/8 size.

Duro 1/2-Drive Large Sockets from "Double Guarantee" Socket Set

Duro 1/2-Drive Large Sockets from Double Guarantee Socket Set]
Fig. 34. Duro 1/2-Drive Large Sockets from "Double Guarantee" Socket Set, with Inset for Broaching, ca. 1929-1931.

Fig. 34 shows the three largest sockets from the Duro "Double Guarantee" 1/2-Drive Socket Set, with sizes (from the left) 15/16, 31/32, and 1 inch. The sockets are stamped with the Duro D-Trapezoid logo on each side of the fractional size, except that the 1 inch socket has only one "D" logo.

The finish is nickel plating.

The larger sockets in the set (from 11/16 up) have a distinctive design with a reduced diameter at the 1/2-square drive end, a pattern mirroring the reduced diameter at the service end for the smaller sockets.

The sockets have a band of cross-hatched knurling at the service end, with the knurling coarse enough to assist with turning a nut by hand. The photograph shows some of the variations in the knurling pattern -- the left socket has a very fine rectilinear grid, and the middle socket has a more coarse diamond cross-hatch pattern. Most of the sockets have a medium-fine cross-hatch pattern similar to the right socket, but at least one socket in the set had no knurling at all, probably due to a production accident.

Early 12-point Sockets

During most of the 1920s sockets were generally made with six-point (or four-point) broachings, but by the late 1920s several companies had started producing 12-point (double-hex) sockets. By the early 1930s Duro/Indestro had begun producing 12-point sockets as well; although the earliest date is not known, we do know that by 1931 Duro and Indestro socket sets were definitely available in 12-point broachings.

The first confirmation comes from the 1931 Sears catalog, which lists several "Durobilt" brand socket sets that are easily recognizable as Duro production. The high-end set featured chrome-nickel sockets in the standard 6-point broaching, but with 12-point sockets available at a slightly higher price.

A second confirmation is even more direct: by fortunate circumstance we were able to acquire an early Indestro Chromium Vanadium 12-point socket set, complete with a dated sales receipt from 1931.

Early Indestro C-V 12-Point Sockets

Indestro 1/2-Drive Chromium Vanadium 12-Point Sockets]
Fig. 35. Indestro 1/2-Drive Chromium Vanadium 12-Point Sockets, with Inset for Detail, 1931.

Fig. 35 shows a group of sockets from an early Indestro Chromium Vanadium Socket Set, dated to 1931 by a sales receipt. All of the sockets shown are marked "Chromium Vanadium" and have a star symbol on each side of the size, a characteristic marking for Indestro sockets.

The sizes are, from the front left, 1/2, 9/16, 5/8, 21/32, and 11/16; and in the back row, 3/4, 13/16, 7/8, and 15/16. The inset shows a typical "Chromium Vanadium" marking.

The sockets have a coarse knurled band around the base, probably intended to help with turning a loose nut by hand.

The socket construction is cold-broached with a bored recess at the base. If you look carefully at the full-sized photograph, you can see chatter marks typically left by cold broaching.

Duro No. 500 Socket Set

As 12-point broachings became standard for alloy steel sockets, Duro/Indestro changed its carbon steel (or "Select Steel") economy socket sets to 12-points as well. Although the dates are uncertain, the two examples below are thought to be representative of Duro and Indestro economy-line 12-point sockets in the early 1930s.

Duro 500 Hex Handle With 12-Point Sockets]
Fig. 36. Duro 500 Handle with Early Hex-Drive 12-Point Sockets, ca. 1932-1933.

Fig. 36 shows a partial Duro No. 500 set, consisting of a Duro 500 hex-drive handle and several sockets. The handle is marked "Duro Metal Products Co." and "Chicago U.S.A", along with a notice for patent #1,788,535, issued in 1931.

The sockets in this set retain the earlier form with a reduced base for the larger sizes.

The full No. 500 set included a ratchet, Ell-handle, eight sockets, and a screwdriver bit, all contained in a distinctive fold-open metal case. (The metal case is the subject of the patent mentioned.)

Indestro No. 1236 1/2-Hex Drive 12-Point Socket Set

[Indestro No. 1236 1/2-Hex Drive 12-Point Socket Set]
Fig. 37. Indestro No. 1236 1/2-Hex Drive 12-Point Socket Set, ca. Mid 1930s+.

Fig. 37 shows a mostly complete Indestro No. 1236 1/2-hex drive socket set, consisting of a ratchet, a hex handle, a screwdriver bit (doing double-duty as the ratchet plug), and 12-point sockets, all wrapped up with one of the clever holders that were an Indestro specialty. Of the pieces here, only the ratchet is marked, with "Indestro Mfg. Co. Chicago, U.S.A.".

No model number is marked, but the set is similar to the No. 1236 set shown in the 1941 Indestro catalog, with a few sockets missing.

Note that the sockets in this set have been simplified to a plain cylindrical form, although they retain the fine cross-hatching at the base.

Drain Plug Sockets

Duro and Indestro socket sets often included specialty sockets with square studs in various sizes, intended primarily for removing drain plugs. These sockets were often unmarked except for the size, but are listed in the older catalogs with assigned model numbers.

Duro [9954] 1/2-Drive 1/2 Drain Plug Socket

The older Select Steel (Carbon-Manganese) socket sets included drain plug sockets with sizes ranging from 5/16 to 11/16, with corresponding model numbers 9951-9957 for Duro and 6651-6657 for Indestro.

[Duro 9954 1/2 Pipe Plug Socket]
Fig. 38. Duro [9954] 1/2 Inch Drain Plug Socket with D-Trapezoid Symbol.
[Chromium Vanadium 1/2 Inch Drain Plug Socket]
Fig. 39. Indestro [3316] Chromium Vanadium 1/2 Inch Drain Plug Socket.

Fig. 38 at the far left shows an example of the Duro [9954] socket, marked with the size (1/2) and the D-Trapezoid symbol. The corresponding Indestro pieces are essentially identical, except that they omit the D-Trapezoid marking.

The newer alloy steel socket sets also included drain plug sockets. Duro offered these in sizes from 5/16 to 3/4 by 16ths, plus the 11/32 and 13/32 (hexagon) sizes for Hudson-Essex applications. The model numbers were of the form "13xx" where "xx" is the size in 32nds.

Indestro offered fewer sizes, just 5/16, 3/8, 7/16, and 1/2 with model numbers of the form "33xx" as above. Fig. 39 at the left shows an example, an Indestro [3316] 1/2 inch drain plug socket marked "Chromium Vanadium".

The Development of Hot-Broached Sockets

In May of 1935 Duro filed for an important patent on their new hot-broaching method for making sockets, and the patent was issued as #2,027,922 in January, 1936. The patent document has an excellent discussion of the prior art (cold-broaching) and the advantages of hot-broaching, and is highly recommended for anyone interested in the how and why of such things. Among the advantages cited are the increased strength of the sockets, the smooth broach free of chatter marks, and the relieved lip at the drive end that allows for easier connection to a drive tool. (The patent even mentions the attractive "scalloped" appearance of the metal pushed ahead of the broach.)

Early Duro-Chrome 16xx-Series Sockets

[Early Duro-Chrome 1/2-Drive Sockets]
Fig. 40. Early Duro-Chrome 1/2-Drive Hot-Broached Sockets, with Inset for Broaching, ca. 1935-1937.

Fig. 40 shows several early Duro-Chrome hot-broached sockets in the 1/2-drive 16xx series, with an inset at the top to illustrate the broaching details. The sockets are marked "Duro-Chrome" and have a cross-hatched band flush with the surface.

The models and sizes are, from the left, 1628 (7/8), 1630 (15/16), 1632 (1 In.), and 1634 (1-1/16).

A close look at the inset shows some degree of variation in the shelf left by the broaching; in particular, the rightmost socket has a wide shelf with nearly perpendicular scallops. This suggests that Duro was still experimenting with their process at this early stage. The third socket from the left (model 1632) most closely resembles the modern form.

The drive openings of these sockets have smooth walls without the recesses found on modern sockets, another indication of their early manufacturing date.

By fortunate circumstance, it turns out that the hot-broach patent helps to estimate the production date of the Indestro Chromium Vanadium Socket Set displayed on a previous page. On close inspection of the sockets in that set, it turned out that some were hot-broached, while others were of the older cold-broached construction. With the reasonable assumption that production would have quickly shifted to the new hot process, this suggests that the Chromium Vanadium socket set was probably made in the 1935-1936 time frame. The socket set thus gives us a valuable snapshot of the design and construction of sockets being produced at that time.

In particular, we see that the sockets have the tapered walls and knurled base of the later modern sockets, and that the sockets were being finished with a highly polished chrome plating. Part numbers were not yet being used at this time, though, as the sockets have only the size markings.

Modern Sockets

Duro and Indestro must have been very busy during the late 1930s, as their socket lineup developed very rapidly in these years. By the time of the 1938 Duro-Chrome catalog (38M), almost all of the modern series of sockets had been developed, and the sockets had been assigned part numbers. These included the 1/2-drive 11xx, 16xxx, 12xx, and 14xx (deep) series sockets, the 3/8-drive 44xx series, and the 1/4-drive 45xx series.

Duro-Chrome 1/2-Drive Socket Series

[Duro-Chrome 1/2-Drive Sockets]
Fig. 41. Duro-Chrome 1/2-Drive Sockets, Left to Right: 1118, 1618V8XY, 1625, 1622S.

Fig. 41 at the left shows the top-of-the-line for Duro-Chrome's 1/2-drive sockets, the 11xx series with thin straight walls and 16xx series for thin tapered walls. These included selected 8-point sizes (marked by appending an "S" to the model number) and the specialty 1618V8XY socket, the latter made with ultra-thin walls for certain connecting-rod bolts.

The models and sizes are, from the left, 1118 (9/16), 1618V8XY (9/16), 1625 (25/32), and 1622S (11/16, 8-Point).

12xx-Series Chrome-Vanadium Sockets

The 1938 Duro catalog also shows a 12xx series of 1/2 drive 12-point (and 8-point) sockets that it refers to as "Chrome Vanadium sockets"; these are the model numbers that would later become the Indestro Super line. At this time they were marked with "Chromium Vanadium" and the model number, as shown in Fig. 42 below.

[Chromium Vanadium Sockets]
Fig. 42. Duro Chromium Vanadium Sockets, Left to Right: 2812, 1220S, 1234.

The socket on the right, marked 1234 (size 1-1/16 inch) is believed to date to the '30s, as it shows the cross-hatching in use at this time. Later on these models changed to the simpler straight knurling shown on the middle and left sockets. The 1220S number on the middle socket indicates an 8-point broaching.

The leftmost socket is a 1/4-drive 3/8 socket and has model number 2812; although this model is not shown in the 1938 catalog, a later Indestro catalog lists 2812 as one of the Indestro Super 1/4 drive line. This again confirms the continuity of the "Chromium Vanadium" mark with the later Indestro Super line, with a likely "Super-Quality" intermediate.

44xx 3/8-Drive Sockets

The 1938 Duro catalog lists the 44xx series of Duro-Chrome 3/8-drive sockets, but for some reason this drive size was not yet available in the tapered thin-wall format offered in 1/2- and 1/4-drive. Instead, the catalog illustration shows these sockets as having narrowed bases and straight walls. An example of this can be seen in the leftmost socket of Fig. 43, marked as Duro-Chrome 4420 with size 5/8.

[Duro-Chrome 3/8-Drive Sockets]
Fig. 43. Duro-Chrome 3/8 Drive Sockets, Left to Right: 4420, 4414, 4422.

The tapered-wall format was made available later, as the middle and righthand sockets in the figure show. These sockets, marked 4414 7/16 and 4422 11/16 respectively, both show the same fine cross-hatching that was used for the 1/2-drive 11xx and 16xx series (see the Duro 1/2-Drive Sockets above.)

The middle socket shows two other interesting details: the socket is in unfinished steel rather than polished chrome, and it has a small "G" next to the model number. I have one or two other Duro pieces that also show a "G" marking paired with unfinished steel, and this certainly suggests that the "G" may indicate a 1945 production date when government wartime rationing was in effect, as with the "G" date code for Snap-On tools.

Duro-Chrome also offered 1/4-drive sockets in the 45xx series, with tapered walls and a cross-hatched band, but these are not illustrated at this time.

The D-I Sockets: Wartime Production?

There is one other group of sockets that may be seen occasionally, but which don't fit any of the markings previously noted above. These sockets closely resemble the Chromium Vanadium or Indestro Super sockets, with knurled bases and tapered walls, but are labeled with "D - I" instead of the expected Indestro brand.

[Sockets Marked D-I with 12xx Part Numbers]
Fig. 44. Sockets Marked "D - I" with 12xx Part Numbers.

Fig. 44 at the left gives some examples of D-I sockets; many other examples are available, in drive sizes from 1/4- up to 3/4-drive.

Based on the close physical resemblance of the sockets, the "D - I" notation almost certainly denotes "Duro-Indestro"; for example, one set of D-I sockets was acquired in a metal box with a Duro-Chrome sticker. In addition, the 1/2-drive D-I sockets usually have a 12xx part number (as in the figure above), offering further evidence of the Duro/Indestro connection. But why would they have been marked differently?

Our first assumption was that these might be very early sockets, produced before the standard markings were developed. This fit nicely with the observation that all D-I sockets found to date are either of unfinished steel or have a thin cadmium plating, rather than the polished chrome in use after the mid 1930s.

However, subsequent findings forced rejection of this idea. One piece of contrary evidence is that all D-I sockets found have been hot-broached, which based on the patent date pushes the production to post-1935. Also, the presence of the 12xx part numbers implies production in the late 1930s, at which time the standardized marking and polished chrome finishes would be expected.

Another possible explanation is that the D-I sockets were intended as an economy brand, made with cheaper steel and finishes. But if this were the case, they would have been given new part numbers; reusing the 12xx part numbers would be too confusing to customers.

After the above considerations, one explanation remains that appears to fit all of the observations: the D-I sockets represent wartime production, when rationing of key materials (including chromium and steel itself) forced companies to work with lesser quality steel, or at least forego polished chrome finishes. The D-I sockets might have been produced under a special contracts, as with the Plomb "Wright Field" WF-xxx series tools, or they might have been regular civilian production made to lower standards or with a plain finish. In either event, it would have been desirable to mark the items distinctly so that they wouldn't affect the reputation of the well-known Duro/Indestro brands.

Indestro "Super Quality" 1/4-Drive Socket Set

This next figure provides one more look at the D-I sockets, in the form of a 1/4-drive socket set.

[Indestro Super Quality 1/4-Drive Socket Set]
Fig. 45. Indestro "Super Quality" 1/4-Drive Socket Set with "D - I" Sockets.

Fig. 45 shows a 1/4-drive Indestro "Super-Quality" socket set, in which all of the sockets (except one) are marked D-I. (The exception is actually marked "4511 Duro-Chrome", a standard model number for a 1/4-drive 11/32 socket.)

Note that the Indestro logo sticker includes "Chrome Vanadium Steel", suggesting that at least for these tools, the steel quality remains uncompromised. Otherwise, the tools have a dull industrial finish and the box is a drab olive color, which would be consistent with wartime production.

Post-Modern Sockets

Sometime in the late 1950s or early 1960s Duro began making changes to their socket designs, with the result that some of the finer features of the earlier sockets were lost. These changes may have been driven by the need to reduce costs under competitive pressures.

Duro 1/2-Drive Sockets Showing Progressive Changes

[Duro Sockets Showing Progressive Changes]
Fig. 46. Duro Sockets Showing Progressive Changes, from Left: 1619, 1618, 1626, 1634/1234.

Fig. 46 shows a progression of socket design changes, starting from the reference socket on the left, a Duro-Chrome 1619 with tapered walls and a finely cross-hatched knurled band.

The second socket is a Duro-Chrome 1618 and still has tapered walls, but the fine cross-hatching has been replaced with a simpler straight knurled band. The illustrations in Catalog 37-M from around 1961 show that this change had been made by this time.

The third socket from the left is marked as a Duro-Chrome 1626. Note that this socket now has straight instead of tapered walls, thereby losing what had long been touted as an important feature for working in tight places.

The final change is shown in the rightmost socket, which similarly has straight walls, but is now dual-marked as both a Duro-Chrome 1634 and a Duro-Indestro 1234.

That completes our discussion of Duro/Indestro socket identification. The next sections will present examples of Duro and Indestro tool production, including information on their early tools, the development of alloy steel tools, drive tools and sockets, and specialty tools.

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