The Snap-On adapter sounds like a fix for the odd drive size of 9/32 that was a military request during WWII. I too have several of the 9/32 to 1/4 inch drive adapters made by several tool makers. Plomb also made that 9/32 drive size. I don't know how common they are as i stated, i have several. I am sure a lot of them got tossed because nobody knew what they were for. You may have some Plomb 9/32 sockets there.
EvilDr235