I think there are three possibilities.
In the first case, the lock could have been imported in a finished state and engraved here. This would have required Dreppard to have annealed the lockplate, engraved it and re-hardened it since, as far as we can tell, lockplates were hardened before they were shipped. Certainly this would not have been beyond the abilities of most American gunmakers but they ran the risk of having the lockplate warp in the hardening process.
At a much later date, by the middle of the 19th century, we know it was commonplace for American gunmakers to order batches of locks with their names on them but I've no evidence it was done this early.
A third possibility also exists. We know this was done later, towards the middle of the century. A Birmingham lock filer / finisher emigrated, thinking he'd easily find work in the Philadelphia area. He discovered that, with no local lock making industry, there simply was no work for someone in his very specialized trade. He eventually imported locks in the "soft" state and finished them himself, claiming to be the maker. In the context of 19th century business that was only a slight exaggeration. The 19th century had no "truth in advertising" laws so we cannot always take the statements made in advertisements at face value, though even when questionable they offer important clues to what was going on. By 1810-1813 exactly the same thing was going on in the London trade, with many gunmakers simply ordering in guns that they finished themselves or simply put their names on. Around 1810, when the London gunmakers pushed for a law requiring that all guns be marked with their city of origin, the B'ham makers protested that most of the "London" barrels and locks were made in B'ham! Even the London trade had given up to the economic power of B'ham.
I have noticed that both Pennsylvania and New England rifles almost always have a much better grade of imported lock. This is certainly rational given they were more expensive guns to begin with and, while changing a lock was possible, it was never as easy as just plugging in another identical one. I also suspect that the customers for rifles were much more likely to be demanding of quality. In a system where every adult male had to have a militia musket there had to have been a large number of men who simply wanted the cheapest gun that would "pass muster." I see no reason to apply that reasoning to rifles, nearly all of which must have gone to men who were actually interested in firearms and wanted the best they could afford.
As to the question of making high quality locks in America... lockmaking was extremely well organized and sophisticated. Even the lockmakers we have the names of, and there are quite a few of them, were owners and organizers utilizing the services of "cock forgers," "lockplate forgers," "pin [screw] makers." etc. Something as simple as drilling and threading the blind hole in the frizzen spring of the lock you illustrate probably required several specialized jigs and fixtures and perhaps a man whose full time job was nothing but threaded blind holes. This is a degree of specialization that wasn't possible in America until at least the 1840s. One of the most important factors in the American creation of modern mass production was the dearth of highly skilled workmen, a commodity Birmingham had an almost limitless supply of. Also, when we do encounter undeniably "American Made" locks like those on Bedford County rifles, they look quite different from the imported variety.
