You're on the right track to feel suspicious about it. I can't tell whether this is a restock or a heavily sanded and refinished original stock - My gut says restock, given the extra hole in the sideplate. The stock almost has the look of black walnut too. The piece as a whole is not 1740s, HH is closer with something more likely 1780s, or maybe even a smidge later. I believe the steel and the brass have all been cleaned. The sideplate and barrel look like they fresh "brass black" applied and rubbed back, and the iron parts have a wirebrush finish. A lot of folks today use brass black to age brass, but it doesn't actually have the same appearance as aged brass. I've seen locks like that on blunderbusses before, especially later flint period guns. If you're looking to buy one, keep your eyes open on gunbroker and other online auction sites - they come up now and again in original condition for similar price.
-Eric