I'm heading in the other direction. The absence of the "not for ball" and the 12b and/or 14 on the flats make me think it is not in the pre-1875 window, but rather on the other side of the 1875-1887 threshold. If I knew more about what I was looking at, who was alleged to have made it, etc, I'd be more confident.
Pre-1875? Weren't they still beating each other with clubs at that point?
All jokes aside, how many breechloading guns existed in Great Britain pre-1875? Lang had a patent. Lefacheau (sp?) although French, may have licensed something. Manton? Purdey?
Odds have to be in favor of a later date unless you explain that this a pinfire or something else in the novel/interesting category.