Researcher, I would put the date for the 2 1/2" .410 cartridge a little before that. As far as I can ascertain, at least on this side of the Atlantic, it came about via BSA making a cheap bolt action single .410 built on a Lee Speed .303 action that they re-barrelled and restocked; Kynoch then made the 2 1/2" cartridge for it. I think that it was first introduced in 1913. I don't know when U.S. manufacturers took it up but I would suspect that it would not be long after. I used to own one of those old BSA's but foolishly parted with it many years ago. Lagopus.....