Help Identify This Drain Socket

[Provincial]

Friday I picked up a 1/2" square drive socket with a male 11/16" square working end. I assume it is a drain plug socket. It was covered with a thick coat of old hardened cosmolene, and the finish indicates WWII era. It seems well-made.

The only identifying feature is the knurling. I call it "dot knurl" because rather than criss-cross grooves, there are criss-cross lines of depressions. These show well in the photos.

I think the plug must have been at least 1" pipe size in order to accomodate that size square recess. I didn't see anywhere on the internet that showed dimensions of hollow square pipe plugs.

 

[Provincial]

Thanks! I thought the knurling was distinctive.

Now to determine if it was a standard part or special.

 

[Provincial]

Given the lead by Ricky Joe, I looked at Alloy Artifacts and found a reference to the drain plug sockets:

"Fig. 44. Duro [9954] 1/2 Inch Drain Plug Socket with D-Trapezoid Symbol.
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.

Fig. 44 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 last sentence indicates that my socket is probably an Indestro. The question now is what might be the military connection?