Interesting supposition. I just googled .275 Rigby and got a page full of responses.

One was: The .275 No 2 Magnum, also known as the 7mm Rigby Magnum Flanged and the .275 No 2 Rigby, is an obsolete centerfire rifle cartridge developed by John Rigby ..

Here's another: W.D.M. Bell Edition ~ .275 Rigby - Cabela's
https://www.cabelas.com › Home › Used Guns
.275 Rigby from www.cabelas.com
... signed by all the Rigby gunmakers involved in production of the rifle. The rifle is available in .275 Rigby, as per Bell's original rifle." Show more.

another:Description. Hendershot's is your best source for 275 Rigby Ammo……Yes, we use properly headstamped brass manufactured by Hornday!