There was no requirement to stamp chamber length on Brit guns until 1925. Odds are very strong that one is indeed a 2 1/2" gun.

I think the value of that gun depends on where you are. I made a very good buy on an Army & Navy sidelock nonejector in England a number of years ago, simply because the Brits think sidelock nonejectors are somewhat odd ducks. Any Brits present can speak up, but I think the price would take more of a hit in England because of the lack of ejectors than it would in the States. Canada . . . not sure how it would work up there.