Okay...Andy has come back to me with this -

tac,
The first thought that comes to mind is that Ketland only made the lock not the whole rifle.

While looking through the book "English Guns and Rifles" by J.N. George ... I come across several references to "Screw Barrel" rifles and "Screw Plug" rifles ... both systems seem to have been popular to experiment with during the flintlock era...
A picture of the rifle would be a big help....
I would be happy to look more into this rifle as more information is available.
Andy

Over to you.

tac