This is a NEEDLE-fire rook rifle --- Rissack's Patent (most frequently encountered marked "Rigby, Dublin) --- uses a paper cartridge with the primer at the base of the bullet. Pin pierces "cartridge" and hits primer. I would expect it to be about 100bore (about .30calibre -- 7.5mm or so) so very little recoil.
Venables would have been the RETAILER, not the maker. Probably Birmingham proofed.
It would fit one of my collecting themes --- rook rifles (of course!) . I have seen a couple of PISTOL versions which would fit the second of my themes - -"unusual" single-shot pistols.
I also collect British revolver aand their copies -- -Webley, Adams, Tranter, Deane-Harding etc.
Luckily I have Home Office Authority for ANY type of pistol, including those now classified as "small firearms" in the UK.
Jim H.