Thanks, that explains the size in your hand. It is a but plate, I recon it's brass ?. I had a early PA flintlock custom made years back, I think it's 1740 to 1760's style , it has a wide butt plate made of steel,almost like the one you have. Then by the end of the F & I wars and our revolution they slimed down and started using the more Roman nose butt stock as we know. Mine
has a more straight stock than a full Roman nose. If it's from an original , it's a great find. if it's not ,it's still a great find.