I think you are spot on. I wonder what the 51 would have stood for.While a lot of rifles shared some parts back and forth. My first few minutes of googling have found this one. What looks really good to begin a match is that short rat tail on it.
Richmond Rifle Musket, Patch Box | Civil War Arsenal
www.civilwararsenal.com
It's probably a part number code if i had to guess.I think you are spot on. I wonder what the 51 would have stood for.
If you look at the way it the tail is shaped and the curve on the butt of the rifle is shaped the side with 51 stamped on it would have been on the inside. Which makes sense for a part number to meWell if the rifle was a military or militia like modern day, they are doled out with a number assigned to a specific person. Possibly a part number, but if it is a PN wouldn't it be inside instead of on the face? In any event, very cool find for sure.