The barrel with its breach and turnngs makes me think the barrel is british, along with the brass hardware, but the british / american british style muskets are not an area of expertise for me, either.
I was always leery of paying alot for a gun because of rev war markings, too many people know how to fake markings and age them. I have a 1768(?) Charleville that was converted in the US, that is unmarked - which I prefer! At least I know it has no fake marks. I am not suggesting that the musket posted is fake in any way, it may be right as rain.
The sideplate looks funny to me, does not look british, but also does not strike me as an early french plate, but maybe someone else here knows.