At the transition from octagon to the round section there is a "wedding band" on an original hand made barrel it would have been made using hand files, yours is lathe turned.
I manufacture black powder barrels, I know what a modern made barrel looks like as opposed to an original hand made barrel.
And modern replacements or not, those wood screws are not how a lock would have been attached to the stock on an original or even a quality replica of one, it would have had bolts passing through the stock from the other side of the lock plate at the front and top center of the lock that threaded into the lock plate.
Yours has wood screws at the rear and bottom center holding it from the lock side. Sorry to disappoint you but your antique isn't an antique, it's a low cost modern replica, does it even have a touch hole in the barrel? If not then it's a non firing replica.
If it was found buried in the mud of a river bank, I suspect someone buried it there and not very long ago, riverbanks are notorious for destroying wood and wrought iron in a relatively short time.