I believe the back action locks were popular in the 1840-1860 time frame. These were often held on with just one screw thru the stock, and sometimes with a small wood screw at the tail of the lockplate.
Although there were some back action flintlocks, the pan and frizzen were often a part of the barrel. This barrel doesn't look like it had anything like that on it before becoming a precussion gun.
The barrel tang, the flats on the side of the barrel breech, the trigger guard and the butt plate all look military to me.
The large hole for the side drum could easily have been drilled and tapped into a old flintlock barrel.
I'm not sure about the other letter, but the P was sometimes the symbol for passing a successful Proof Test on military guns.
The stock doesn't look like anything I have seen before. Especially the large cane handle shape of the wrist.
It was probably custom made to tie the military? pieces together?
All in all, it's a very interesting old gun.
To make a wall hanger you might think about these ideas:
A replacement drum can be bought, and although it probably wouldn't fit the threads, it could be epoxied in place to fill up the hole.
If you make a cardboard or paper pattern of the lock mortice, you could make a dummy lock plate, and hammers are available in a lot of styles from the lock companys and from Dixie Gunworks.
It would just take a lot of grinding and filing on a piece of 3/32 thick steel.
:thumbsup: