Your point about this handguns age is interesting. When I spoke to an agent at a BATF Field Office concerning FFL transfers of pre-1899 Antiques, I was told that the ATF does not consider pre-1899 Antiques to be firearms in the legal sense, and that they should not even be entered in an FFL's bound ledger book.
Edit: I'd be interested in seeing a picture of the barrel breech to see if it is perhaps a pinfire rather than a rimfire. Not a typical pinfire hammer, but it seems odd that the tit on the hammer would strike the circumference of the cartridge rim. Here's a 9 m/m pinfire cartridge:
