How is the lock marked? Does it say simply "Ketland" or "Ketland & Co." That may help in dating it. None of the German-style rifles supplied for the Revolution and earlier were Ketland products but German-style rifles enjoyed a brief spurt of popularity in Britain, perhaps as a result of their use in America. The F&I War is almost certainly too early for a Ketland gun and it is very unlikely that Thomas K was supplying loose locks that early. He could, however, have made the whole gun. I'd love to get some good high res photographs for my Keltnad book. I'm on Chapter 3 right now, dealing with the early products of Thomas Sr. If the lock is simply marked "Ketland" it would fit in that chapter.
Proof marks would be interesting but they aren't necessary. British law at the time did not require proof outside metropolitan London. Most guns were proved but for some reason, rifles seem to lack proofs fairly often. It almost certainly pre-dates the Birmingham proof house so if TK make it, and it was proved, it would have Birmingham private proofs or London proofs. In both cases, this early, they are nearly never under the barrel. The whole purpose of proof was to be visible.