Ok to answer some of the questions. The lockplate appears to be original, but there is no way of really telling without a lot more detailed examination, and with the rifle in the fragile condition it's in I would not be willing to remove any part. I do see what you mean, at places the plate does appear to be smaller than the mortice in the pictures, this was not quite as appearant in person, the front end of the mortice appears like it was damaged. The only replaced wood anywhere that I could tell was the repair at the top front end of the lockplate, and it is a different shade of wood. The plate does have some faint "commercial" type of engraving, like the kind that is found on the "Warranted" type plates, that did not show up at all in the pictures. It may not be readily noticable in the pictures but the percussion drum is slanted forward a bit, and the nipple is completely worn away. A peculiarity of this rifle is that, with exception, of the brass sleeved repair, the left hand side of the rifle looks rather well preserved, while the right hand side is where most of the wear and damage is visable.
And on the question about the trigger guard. It is at least 3 seperate pieces joined together.
It would tickle Gary to no ends if this thing was originally made in flint, but I personally am not comfortable making that call. There is also a horn and possibly some other accoutrements that came with this rifle when it was passed down from his uncle, but he has those stored away at this time.