According to the marks it is a 14 bore that is choked made between 1875 and 1887; most probably made to the latter end of that period. If it takes a 12 bore cartridge then I suspect it has been reamed out at some time. I've had a quick scan but can't find a Patent 76 so maybe it's a Use Number. I will have another look but it probably relates to the lock. Lagopus.....