I have a rifle by A B Carpenter, who worked in Paris, and Dolson, Illinois, 1870's & 1880's, that has a brass lock plate. It has remnants of silver or nickel plating on the lock plate as well. It is in the Library & Museum here. If you look at it, you will notice that the plate is cut for a bolster/snail, but now has a drum. That's because it has been set-back about 5" and re-breeched., leaving it with a stubby 47 1/2" barrel.