If anyone needs this I have no use for it, for a few bucks and cost of shipping it's yours.
Found this 9/16" drive ancient socket. The hex is also for a modern 9/16" bolt.
I think I'm officially getting good at spotting random tools and have about a 30-40% success rate at saying something is Plomb-empire and being correct. This was dirty and in a pile of rubble. I couldn't actually see the plomb markings for about 30 seconds of rubbing but I had a good feeling.
The square end is perfectly 9/16, and the milling/grinding that doesn't appear on these tools as seen on AA seems to have taken place after the stamping.
If I were to guess I'd say they got it stamped and for some reason it didn't fit 9/16 female ends, so after it was already stamped with ID markings, they took it to a grinder.
IMO you can tell the difference between factory grindings and owner grindings.
AA:
http://alloy-artifacts.org/Photos/tools/plomb_916mdr_s26_socket_6pt_la_f_cropped_inset.jpg