Jim,
I'm more skeptical than you about this rifle. Do you believe it has an original patchbox? It's a nice box, but I strongly doubt it is original, based on the strange hinge... and TOTAL lack of any small dings, scratches, dents, hits along its edges, etc. I agree somebody had some skill in making and engraving the box, but I'd bet it is 20th century if inspected closely, not a late flint restock with salvaged box. Nothing explains the fresh, clean edges on the forestock's top edges above the side facings in the 2nd picture... no wear, no dings, no real age. I'm also skeptical a modern lock could fit so perfectly in an "old" mortise, despite what the mortise looks like inside. I would expect more variation in the width of the side facing band around the current lock if it was a replacement for an earlier lock, and I do not see that type variation. To me, it looks like the mortise was built around this modern lock. I'd agree a competent person did the work, but the carving is all modern, and the stock looks new, no real dings, damage, slivers, etc. I guess we could say it sat in a closet and was not used, but that's a stretch. This stock with its modern butt plate, oddly hinged and untouched patchbox, odd side plate, strange side facing treatment, etc., etc., just doesn't add up to an old rifle restocked in late flint era and reworked a bit later on. But I like the barrel.
Shelby Gallien