CarlsenHighway:
I might suggest checking out the cartridges offered for sale by Mantons of Calcutta, on page 104 of their 1925 "Centenary Catalogue and Price List".
Right beside the .275 Mauser, they picture and name the .275 Rigby, No.2HV. That's just a couple of years shy of a full century of referring to the 140gr loaded cartridge as the .275 Rigby.
And that's in the hunting fields of the Raj, where arguably most of these cartridges were expended at game.

So although I may occasionally share your frustration at incorrect cartridge nomenclature, I am at a loss to understand this inexplicable dogma surrounding reference to the .275 Rigby.