Having re read the original post O agree if it is marked 2&3/4" case then I think it is safe to assume the gun was originally chambered for a "2&3/4" roll turn over case .Nitro proof did come in in the late 1890's and became the norm after 1904 .
Many guns made for the export market were still proofed for black powders as nitro powders were not readily available and as "home loading" was widely practiced due spasmodic availability commercially bought cartridges .
I don't know if yours falls into the category but I have seen Lancaster's sold as the Colonial model usually with only boarder line engraving , some marked as "The Colindian "or The Coladian"for export to India and Canada .I don't know if there was a "Colmerican".
