All I can add to this is that I have an Eley pin-fire 28 with a headstamp that dates it between 1866 and 1874.
The .410 really has its origins in the 12mm. Walkingstick gun cartridge. I have some brass ones marked 12mm. W.S., 12mm. Canne. (French) and others marked 12mm./.410 on the headstamp. .410 Pin-fire was also available from Eley's before 1874.
I have also found a reproduction of an Eley advert dated 1857 that lists pin-fire .410 and 28 bore and it seems that they were also available for the Lancaster base-fire shotgun so they may be quite early. Lagopus.....
