Quite right in that the Standard length was 2 1/2" back then but extra length chambers could be ordered for Live Pigeon guns and Wildfowl guns. Chamber length was not marked on the Proof Marks back then. The only hammer gun I have from that period with long chambers is a W.J.Jeffrey Live Pigeon gun. It was proofed for black powder when I got it but is now nitro proof. It was marked 2 3/4" with a small mark by the maker and was stamped on the forend loop and not on the barrel flats where there was no indication by the Proof House. Have a look with a magnifying glass for any small mark in an obscure place on the barrel. Slightly later ones were just marked with the letters 'LC' in a diamond mark with the bore size such as '12 LC' to indicate a long chamber; this was a Proof House mark. This mark appeared after 1887. After that chamber length was indicted by shot weight and not shown on the marks with the actual length until after 1925. Hope that helps but as someone says it may also have been later lengthened but if original I would expect some indication by the maker somewhere. Lagopus…..
