It doesn't look to me like a flint lock conversion, there is so much crud and rust that there is no indication of where the frizzen spring would have been, cleaned up you might be able to tell better. All the caplock plates I know about either have cut outs for a drum and nipple, or various shapes for what ever nipple set up the gun maker used. Other than that I can't find a rifle or pistol with that shape of lock plate. It's not a Springfield, Enfield, Mississippi rifle, or any other I'm familiar with. I have a couple of books I can look in, it's just a matter of time. My wife is very sick, so I'm limited on available time, but I'll see what I can do tomorrow.