Small point:
Aside from the debate about the modern day moniker ".275 Rigby".

Period ammunition was seemingly marked simply 7x57 Mauser, .275 Mauser, or .275 High Velocity for Rigby Rifles

I am not at all sure about the original Rigby rifles, such as my early 1930's example marked "Sighted For Rigby Special High Velocity .275 Bore Cartridge Pointed Bullet 140 Grains", having a chamber leade/throat being "revised" and cut specifically (and only) for the 140 grain pointed bullet.
As an aside; the proof marks do not differentiate between "normal" 7x57/.275 and the .275HV.

My Rigby is assumedly un altered and will easily take the standard 175 grain round nose bullet. The bore and throat exhibit only the 90 years worth of careful use, no evidence of a later (ie: fresher less worn looking) "re-throat"

To my thinking it would make no sense in the old days to have a ".275" that would be restricted to only .275HV cartridges.
"If" one were off somewhere in remote Africa, India, East Asia and there were no locally available .275HV ammunition it would be very desirable to be able to use the traditional 175 grain 7x57 ammunition that might be more commonly available? Relatively simple to adjust sighting to compensate.

I must take my Rigby .275HV out one day with both types of ammo and see what difference there is in bullet trajectory out to ~200 yards.

Now this is not to say that modern Rigby rifles do not have leade/throat specific to the 140 Grain loading.

Just my 2¢ (or 2p) worth.
- Mike