What you have is someone's attempt to make a gun out of some old parts... the stock is anyone's guess but I don't think its particularly old. The barrel, on the other hand, is old and I think may be an early product of the B'ham trade... I think (with a big emphasis on THINK) the marks are the earliest form of the B'ham private proof marks and are probably associated with the partnership of Farmer & Galton... We haven't even been able to absolutely prove that Farmer & Galton had a proof house but we do know one was in operation, on or near property they owned, at the middle of the 18th century and all the circumstantial evidence points to them....
So... the gun itself is of marginal interest but the barrel might be of great interest to a handful of people.