The 275 was merely the British term for the 7X57 Mauser if it was called 275 Rigby. There was the 275 H&H magnum in both belted/rimless and flanged, and is on par with the 7mm Remington or Weatherby (came out about 1911). There was also a 275 Rigby No.2 that was on par with the Mauser.
His rifle was a Westley Richards 275. It was a 7X57 Mauser.