It's not apart of a sight it's a tool to adjust the sights on the rifle.
It was most probably Birmingham made and Murcott would have just stamped has name on it they were usually supplied in the case with the gun.
Hello all
Having shown the photo to a friend who collects antique shotguns amongst other things he has told me that - It’s a vernier rifle sight adjuster.
Interesting that it’s by Theophilus Murcott 68 Haymarket London. As mentioned he invented the first snap action hammerless double barrelled...
