Hi Scot,
Compliments for looking to the old guns for design. If you round the tail on the Davis lock and add a bit of engraving, you'll be close. Best quick way to get there.
This was the Ron Long lock 40 years ago, designed in part for the Hawken rifles that were all the rage then. Looking at the rifle you wish to copy, Jake and Sam may have done the same. If you look at old Muzzleloader mags, Herschel, Steve Davis and all used this lock as their favorite on their squirrel rifles and others. Would be fine for English trade rifles of that period. First made by Ron, then by Silver Fox, then Tom Faux all from CO, and then picked up by Davis.
The frizzen spring is a bit small re the old locks, but that's what Ron designed, and the lock works fine on rifles or pistols. Others and I have used a lot of them for many years, very happy.
So many posts here support "my favorite supplier" or high quality internals, but if the goal is to match old locks on old rifles - their external design, no one outfit has all the right locks for all purposes. Compliments on wanting to get the design right. Lead the pack, don't follow. Learn along the way. Next step will be building your own locks - maybe. And thanks for the link to a very fine and interesting rifle. Bob