That back action lock on a bar lock plate was a typical type for The Midland Gun Company of Birmingham. They mad a 10 called The Farmer's hammer gun, I think, which could be had in 12, 10, or 8. I will have to check if that is the model. They made a lot for the trade and various unusual names can be found on guns by them. Does it by any chance say the words 'Solid bar' on the action flats? Lagopus.....