By the way, Mike Brooks makes a very good point above that is almost always missed by all but experienced collectors. When a lock was replaced in period the touch hole and pan invariably line up correctly no matter how arkward the rest of the inlet looks. They were, after all, interested in function, not appearance.
When is lock is a modern replacement it is almost always lined up as best it can be with the inlet and the pan/touch hole relationship can be way off since the modern faker is working on appearance and often knows nothing about function.
This is, however, a sort of negative proof. Most British export locks were made to standard patterns and sizes as were military locks so if a gun was fitted with a 1728 pattern French lock, another lock of the same pattern will likely come very close. I have the wreck of a very long fowler (51" barrel) that almost certainly had a lock from a 1717 French musket at one time. The gentleman who gave it to me had a loose 1717 lock that fit the mortise almost perfectly but unquestionably did not come from the gun.
The most telling thing to look at is the relationship between the pan and the touch hole. If the touch hole is far at the bottom of the pan or stuck off in the front or back corner you can be pretty darn sure the lock has been fooled with or replaced after its working life was over.
Joe Puleo