Yes, I agree that to be a real possibility though my own feeling (and its just that, a feeling) is that the gun dates about 1790-1800. I thought the lock fit too well to be a replacement and, of course, we can't see how the touch hole lines up. If it isn't just about perfect I'd say you are right and the lock is replaced.
Then, to cast doubt on my own observations, I'll add that we don't generally appreciate how close to identical large lots of imported locks may have been. Their parts were made to specific sizes by highly skilled specialized workmen who usually only handled one part, so a "lockplate forger" would work to a pattern and his work would have had to be easily adaptable to the work of the tumbler maker, the spring maker etc. A case of "musket" or "rifle" size locks were probably very close to interchangeable (at least in fitting a lock mortise) including the side nail holes which had to be drilled and treaded since the lockplates were hardened before the locks were shipped. While it might be difficult to find "a lock that fits" today, it may not have been quite the same challenge when you could go down to Payson & Nurse (Boston hardware dealers) and go through a barrel of locks.
