Mike, that's a beautiful double you have been lucky enough to acquire in an interesting old cartridge that should be very useful for a good range of game today; pig, deer, etc.

My copy of COTW sits handy so I can pick up and read anytime I'm relaxing in my favourite chair. I recalled the old .242 cartridge being listed under the British cartridge section so checked again when your post came up just to see exactly what sort of ballistics the old girl produces.
COTW lists the cartridge as the .242 Rimless Nitro Express which was originally called the .242 Manton after the Calcutta based company of J. Manton & Co for whom the cartridge was developed by Kynoch in 1923.

I note you also refer to the cartridge as the .242 Vickers. Out of curiosity, where did the Vickers nomenclature come from?

Vickers did make up Mauser 98 rifles for various cartridges, the .242 Nitro Express/Manton being one and also the 404 Jeffery as another example, but the cartridges were not themselves given the Vickers nomenclature?