Thank you Tony!.

Regarding the origin of the .577/500 Magnum:

When looking in the book "Eley Cartridges. A history of silversmiths and ammunition makers"(2006) written by C.W Harding the Eley cartridge had the .577/500 in program from 1885.
They started with a .577/500 BP Express from 1885-1909...then a Magnum BP.577/500 Express from 1892-1919 and a .577/500 Nitro magnum Express from 1904-1905. The latter might be the nitro-for-Black version.
I will have to investigate when Kynochs started their production of the .577/500 3 1/8".