Leon,
Did you get the patent drawings?
It is quite a short patent and simple to understand what the little button does.
I really feel that you could reverse engineer a very acceptable model for the 1875.
The very first single shot rifle I ever built (30 years ago) is what I named a Sharps 1876 model.
Not the best picture. Old picture. I have since engraved it a but don't have a pic of that.
I used this rifle for years as a competition gun and fired it thousands of times. The barrel was in the white in this photo as I replaced it once and that is the new barrel. Half round/oct.
It is in 40-70 Sharps straight firing a paper patch bullet.
I wish you the best on your 1876 project.