From 1993-1998, I worked in an aluminum founder. We made parts for the big 3, cat, detroit, etc. All finished casting atleast had a date stamp as a minimum. The tool was a simple hand stamp similar to the one pictured above. The tool crib carried the single stamps that was taken out a simple cotter pin, replaced with the new # or #'s, then the cotter pin was reinstalled and was set to go. The stamps would wear out, and the stamp holder would sometimes be loose so the stamps would wiggle around from general usage. When stamping, the rock back and forth leaving a dull or 1/2 stamp. Line workers didn't really care, just another step in the process.
I'd say double stamping, etc, was common. Not getting a good stamp the 1st time, then trying again to make it more legible. Or, inspection dept, (which was the dept. I was in) would have to look for things and re-stamp ourselves. Sometimes, with an older stamp set because we normally didn't (or shouldn't) have to use it that much.
