You put it on your key ring, when you lose the keys, someone finds them and drops them in a mailbox. The mailman delivers them to the insurance agency, who IDs the owner by the number embossed on the tag.
There are many varieties of these. They are from the 1940s thru 1970s.
I can imagine that our "modern" USPS could not deal with that at this point in time.
Nice find sir!