One thing about it I'll bet Stan Baker could give you accurate bore dimensions.

A buddy at my gun club has two Bentley and Playfairs a box lock like that one pictured and a hammer gun both are nice quality guns.
I think his box lock is a Webley Screw Grip Action.

PS...Ky Jon you really think $300 is too high. I sure hope you get a camera for Christmas this year....we can't wait to see pictures of all your project guns.