That's a very interesting gun... I'd say one of the three oldest Ketland guns I've see thus far. I can't tell much from the pictures but does the lock read "Ketland" or "Ketland & Co."... I say that because we know that TK Sr was in partnership with William Walker as early as 1785. I'm not certain that means very much but its an interesting point. Walker's name never appears on any Ketland gun until much later. I suspect the date range the previous owner suggested is right on, which is especially good work as nothing in print and very little on the internet would help.
TK Sr went into business in 1759 or 1760. He was only 23 years old at the time but he appears to have set up shop in his own name by 1760 without serving as a journeyman in someone else's shop. There are a lot of possible reasons for this, not the least being that his father died in 1759 and he inherited some money which probably put him in a position to lease premises etc... The original shop was in Lichfield Street.
I think the most important salient point about this gun are the convex lock surfaces. TK seems to have adopted flat lock forms quite early on. Convex locks are defiantly far less commonly seen. That said, even at this point it is unlikely he actually made many of the parts. The B'ham trade was in full swing this early with specialists supplying everything. He may have finished parts and almost certainly assembled them into finished products. Unlike any of his sons, he actually referred to himself as a "gunsmith."
By 1775 he had enjoyed enough success to move to a higher rent neighborhood in what eventually became the "gun quarter" though when he moved there is was a completely new development aimed at successful tradesmen.
If you can get some good high resolution photos, I'd love to include it in my Ketland book.
Cheers,
Joe Puleo