Thanks Argo44 for Diggory's helpful chart which I've saved.
Game Guns and Rifles, percussion to hammerless ejector in Britain, by Richard Akehurst (1969), states: "The introduction of choke boring in the 1870s caused the issue of new (proof) regulations in 1875". He then illustrates the same Birmingham proof house markings as on this hammer gun; "8B 10M", the crown over V and "NOT FOR BALL".
Apart from one firing pin return spring needing to be replaced, this hammer gun is in remarkable order. Interesting that it came with 60 or so 3,1/4" Eley-Kynoch cartridges, but the proof marks says it's only 3" chambered. I'll shortly be dropping a New Zealand $1 coin down the chamber to check this. The forward 8-Bore chamber is a nominal 23.2mm. The coin is 23.00mm.
It's currently at the gunsmiths for a full strip and clean. The initial inspection showed the locks look great.
