The "SWP" is a challenge, though. The British typically put the regiment and unit on the plate or the tang of the butt-plate. But the Americans grabbed up Besses for their use as well. And Besses were in use in North America from the 1740's through the Mexican War and Civil War.
The 8th Regiment of Foot fought in the U.S., but not the 12th. (Assuming this was found in the U.S.)
Steve
Looks indeed like a Brown Bess thumb plate. You know on the Brown Bess book above with 8 over a fraction of 37 does not mean it was the 37th REgt of Foot but probably 37th soldier of the 8th company. This one really is over marked and only the soldier that did the work knew what it meant. Rev War? War of 1812. later ? It would help to know where it was found and then research possible the units in that area if possible. Not long ago there was a bayonet on Ebay listed as regimentally marked that brought big money but the markings were soldier issued ones. Someone paid the price for that one. Good luck. Gary
Gary
Had an expert state that he's seen these in the past and are from Sir William Pepperell's Regiment of foot (the 51st) from the 1755. It's from a Long Land musket , but an earlier type.