This is back from Dr. Nicholas Harlow which is a day late;)
Thank you for the photograph. Looking at it, I don’t believe that the gun dates to the 1860s. If you have the second edition of Donald Dallas’ history, you will see a similar pair (if not the same gun, actually) on page 166. They are a semi-hammerless design, a very few of which were built at the end of the 1870s as hammerless guns started to take over the market. Unfortunately, it isn’t entirely clear what mechanism they were built on.
I hope that helps.
Yours sincerely
Nicholas Harlow
The term "semi-hammerless shotgun" seems to be a misnomer, at best. Typically, it refers to a few guns of side-cocker design that have internal hammers.
