Help needed with markings on US-made 1/2" sockets

[theHeatingEngineer]

I found some 1/2" sockets in my late father's toolbox which are marked "Made in -U.S.A-". One is also marked "SW60-D-1-1/2". The others have a different number in place of the "60". There is no maker's mark but they look and behave like decent pieces of steel.

The sockets are probably pre-1963 so that's fairly 'old' 

The sockets are tapered which I have never seen before. From the drive end to the open end it reduces in diameter by about 3/32" which on a 1.1/4" long socket is a noticeable taper.

I was musing on the problem while on my daily walk and had an idea. Maybe the "60" is the number of 1/64" across flats (?) which makes this socket 60/64" = 15/16"AF. My vernier measured 0.935" whereas 15/16" = 0.9375". My dad had this group of sockets identified (painted) with the colour used for all his Whitworth sockets, which made me wonder if the SW is BSW without the "B(British)", making it "Standard Whitworth", but I don't see how that relates to the 15/16" AF...

Any leads would be appreciated.

[Lewill2]

I have a lone 1/2" drive Industro Super socket that is tapered similar to that but the markings are totally different than your examples.

[Notlobster]

Duro/Indestro

http://alloy-artifacts.org/duro-indestro-p2.html#d-i-marking

[amecks]

Found a comparison chart for wrenches - only lists the various British, US and Metric.  Kind of annoying that this comprehensive list does not include 15/16 US wrench opening, but we already eliminated that with your your vernier measurement.  The wrench opening for British Standard 9/16 and British Standard Whitworth 1/2 equals 0.9345" - which is only a half thousandth of your measurement. 
Use this chart to confirm some other size sockets to be sure...
http://www.vintagemg.com/ArticlePDFs/WRENCHES.pdf

[d42jeep]

D-I sockets were made by Duro/Indestro during WW2. I suspect that your socket could have been sent to the U.K. as part of Lend-Lease or later since I’ve never run across one marked like that in the US. More than one supplier sent Whitworth tools across the Atlantic during that time period. Since my primary interest in vintage tools is WW2, I have several sets filled with D-I SAE sockets. The examples below from top to bottom are 1/2” drive, 3/8” drive and 1/4” drive.
-Don