This rifle is very similar to my Williams and Powell double in .577 Snider (posted somewhere on this forum, as was a 12 bore) except for the cheekpiece
Non rebounding locks, fences, engraving, finger rest all virtually identical

I could not see the maker's name (Hugh Snow, he is not in Brown's British Gunmakers)
Probably by the same Birmingham contract manufacturer with the seller's name put on

As Daryl writes it could have been based on a 12/13/14/16 bore barrel, to compete with bore rifles and the "express" rifles becoming popular
It would be interesting to know the bore & groove diameters

Quite early, mid 1870's I would imagine

Still, a very interesting addition to any collection

TH44