What's up with the Benjamins?

Ever since moving to Las Vegas, I've noticed that a substantial fraction of $100 bills have odd little stamp marks on them. I scanned in close-ups of the ones that happen to be residing in my wallet at the moment. Most marks are found on the backs of the bills, including all except the odd little crocodilian in the last photo here, which is on the front.

Actually, I spotted a couple of such marks even when I still lived in Minnesota, though not nearly as frequently as around here. I've also occasionally seen them on 20s, but they are much more common on 100s.

Surely somebody out there can explain this oddity to me definitively, or at least posit a plausible theory. It seems most likely that they are institutional stamps, added either at banks or at casinos, perhaps to indicate spot-checks for authenticity or to mark that a bundle of bills has been counted (though when the bills come back through in re-circulation, the latter would tend to confound things, so I don't really think that's it).