Lovely piece Fred!!
The Twigg and the Chambers Ketland have very similar architecture. The first is just noticeably bigger. When you are thinking about using one lock or the other for a build, draw some pictures. Draw the barrel on a nice big piece of poster board. Take a tracing of the lock plates from the locks you think you'd like to use, and lay them on the barrel, pan adjacent vent location. See where the back of the fence lines up, and see where the forward lock nail is going to pass. This you want as close to the centre of the forward end of the plate as possible, and just clearing the bottom of the barrel at the same time. Now draw in your ramrod hole, in this case no more than 3/8" and possibly, only 5/16" for a nice tapered rod. That'll tell you where the bottom of the stock will end up. Does this make the stock too deep through the breech? Your drawing will answer your questions for you.