CollectingSnapOn.com

Adding to the speculation: I would assume Snap-On manufactured large numbers of any one tool- socket, extension, universal, etc.- in a 'run'. Set-up of machinery to produce any particular size is time consuming.. so they make a large 'batch' to supply anticipated need. Assume they might make all 3/8" drive, 1/2" sockets for a day, two days, a week.. whatever. I would think the logo and size stamping would all be done by machine. Left-overs from one year would be stamped and put in a set the following year..?? That wouldn't account for a triple stamp, unless production of one size carried the demand through a second year.
Snap-On also had errors in production, as any company of the day, that could account for some re-strikes. One from my collection is this 1932 1/2" Drive Universal- first stamped 9/16", then overstamped the correct 7/16".

Thanks for the thread, snapmom!

I have an example of a 20's triple stamp socket with 28, 31 and 33 date codes. The warranty/sale date concept makes some sense, but only for double stamps. Any one seen more than three?