The 275 Bore Rigby Special is the British label, or version of the 7x57mm Mauser, other names are:
"275 Cervorum", and 275 Bland (for Thomas Bland).
It is the same case as the 7x57mm Mauser, but the loading may be different in pressure, powder and projectiles.
The cartridge was adopted by the John Rigby Company in 1907.
This is the cartridge the made  famous by Walter D.M.Bell .
Like it or not, Mr. Bell was reported to have killed 1,000 elephants with this  cartridge.
This is a good deer hunting  cartridge, but for elephants? A fairly hard to find Head Stamp.
$ 3.95 per cartridge
limit 10 per customer
Pictured on the left is the
cartridges available for sale.
Introduce by Hornady USA
in a limited run in early 2015.

275 Rigby

with correct head-stamp,
loaded with a 140 grain
Jacketed Soft Point.
British Cartridges
275 Rigby