Why is my money infested with little round stamps?

[quakerboy]

I recently had a car smashed. The insurance paid up, which resulted in my having far too temporary of access to far more cash than I have seen in a long time.

While posessing this money, and considering taking some sort of "money bath", attempting to do my best Scrooge McDuck impression, I noticed that a significant minority of the bills had been stamped with odd little stamps on the back.

So. Whats the deal with that?