Hello Joe & all,
I have been looking and mulling over this gun for a while now, and am No authority, but think the stock and carving have a somehow an early N American look to them.
What I Think may be the case, is an earlier (1750-60 Brandenburg) piece possibly with a broken or missing lock, was re-stocked with original furniture and had a 'new' lock fitted at that time.
There is a "However" though! The drum conversion to percussion appears Continental.
I think we can all think up scenarios of how this alteration could possibly take place, but as we should deal with facts, I'll leave it at that.
For what it's worth, I have a Germanic smoothbore, Zella marked barrel, (Near Suhl) that has very similar carving behind the tang. This gun appears largely from the 1760-70 period, with an earlier (17th C) barrel.