The hammer in the picture looks very much like mine. But the firing pin screw and the curved edge of the lock plate are different. The receivers, and triggers are very close. As you say I'll probably never know for sure. The lack of a 12C inside a diamond and the NOT FOR BALL mark narrows it down to 1875-1886. I know it was proofed in Birmingham, and it is a bar lock action, chambered for 2 1/2" ammunition. Maybe someday someone will run across a gun with the angled edge to the lock plate, which seems to be a unique feature of my gun, and we can stir up the conversation again. I've learned a lot considering. Many thanks to all.