found a relevant quote from 'Snapmom', who also believes the military 'theft deterrent' story is incorrect:
Snapmom
"08-23-2009, 09:40 AM
Snap on started 9/32 in about 1925. Industrial 9/32 dr was made after the 40s, maybe into the 60s. These have model numbers like PM***. normal production stopped in the mid 40s. 1/4 dr rats were made in the 30s, and are very rare. 1/4 sockets and other 1/4 dr tools started in about 1944. I think maybe the military thing got started due to the thousands of 9/32 dr sets made for the war effort. The so whitworth 9/32 dr tools are very hard to find, only a few known sets."
The theft deterrent idea is repeated on the Plombtools.com web site as well. I'm inclined to defer to Snapmom's (and luca's) expertise on the matter. It is funny how the internet gives life to a thing though. So long as the internet lives, the story won't die, true or not.
(nice Firestone pics FrankLee, thanks!)