It's not supposition, .275 Rigby is a modern misunderstanding. Rigby never called it by that name, or sold ammo for the ".275 Rigby". There are no original rifles or catalogues listing the .275 Rigby cartridge.

The British gun trade as a whole referred to the .275 Bore, or later to the .275 High Velocity loading.

The most famous users of the cartridge would be Bell and Corbett, who never called their rifles .275 Rigbys in their writing and nor were their rifles marked in that chambering.

American gunwriters have been mistakenly referring to it for the last few years, even people who shuld know better like CRaig Boddington and John Barsness, but British ones usually don't.

Now however it has taken a life of its own, the new Rigby company happily stamping new rifles .275 Rigby and Hornady even make brass with that headstamp. It is now common knowledge that the 7x57 used to be called the .275 Rigby! But its total rubbish. It was never a proprietary cartridge of theirs, now did they sell a rifle ever marked with the name till the new company started up who didnt know any better either.

It's a modern misunderstanding. Sounds very British doesnt it. .275 Rigby. But the 7x57 or the .275 has got nothing to do with Rigby and it was never referred to it as that cartridge name until modern gun magazines and the internet came along.

Essentially people have invented it and overlayed it on the past. It was called the ".275" in the UK or the 7mm Mauser. (As one example from the period, Bell used those names interchangably. )

Drop the Rigby name out of it, Nothing to do with them.*

(*They used to identify their rifles in that chambering as ".275 Bore" or ".275 High Velocity")