Mossy, I see the Birmingham Black Powder proof, Birmingham inspection mark, the S for Servicable, and I think I see SP, but am not sure what that is. One question on the 12. Can you measure the bores ? Every once in a while, one sees guns with a chamber larger than usually associated with the bore. If not a 12 ga bore, I don't know what the 12 is. Maybe some better on proofmarks than I can tell.
I believe your gun was made in England, probably Birmingham and probably with imported tubes from Belgium, but we can not be sure of that. It was exported to the US sometime after 1868 and before 1904. Does it have chokes ? If so, to have the marks proper, the gun would be pre 1875.
I assume there are no marks on the receiver flats. Can you take off a lockplate and see if there are marks on the inside ?
