This looks like it may be an extremely interesting gun - one I would love to get good pictures of. The owner is encouraged to contact me about that but PLEASE don't let anyone talk you into "reconversion." Were this gun reconverted, we would refuse to publish it.
First, the proofs... the pictures aren't clear on my screen but they look as if they are "GR" over a Broad Arrow and Crossed Scepters. These are the identical marks seen on the pistols purchased during the War of 1812 by the Board of Ordnance as gifts for friendly Indians. There is an excellent article on this subject that I have around here somewhere... but I'll have to look for it later. I believe there were a number of long guns as well and, if so, they are dateable in an unusually tight time frame.
As to Ketland & Walker... the partnership goes back to at least 1785 and probably earlier although Walker's name was seldom used. He was probably a money man as he always referred to himself as a "merchant", never as a "gunmaker". TK Sr. died in November of 1816 - Walker died in August of 1819. I have reason to believe that the partnership broke up in 1815 or early in 1816 when TK Jr. returned to the UK from Philadelphia. John Adams was probably TK SRs brother in law. He became a partner in the firm but I am not certain when... I suspect around 1813/1814. There are also guns marked KW&A so there is an overlap when all three were partners.
If all this is correct, especially the bit about the proof marks, the gun was probably made in 1812/1813/1814... again, I will have to check some of my data to be more certain than that.
As nice a man as he was - and he was a friend of mine, the Kit R material should be ignored. The available resources for this sort of work are so much better now than they were when he was doing his research that any comparison is really unfair... suffice it to say he made a lot of guesses, some of which were right but more were wrong. He was also the victim of some bad published sources... a trap anyone can fall into.
Joe Puleo