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I think the "Rigby" ID is that of the cartridges loaded with the 140gr bullet at higher velocity than the standard 7X57
It was Rigby's loading for their rifles sighted to that load.
Bell famously used the military ammunition for elephant so it would seem that, if there was any difference in POI, he compensated for that.

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Rigby’s 1924 catalog showed two models of their .275 rifles: Their No.1 was sighted for the standard 7x57 / 7mm Mauser load with 175 gr bullets, while their No.2 was sighted for the 140 gr High Velocity load. The sighting seems to be the only difference. So Bell may have used the No.1 model.
BTW, that catalog listed a No.4 model in .350 Magnum and a No.5 in .416 “Big Game” too. None of the cartridges were called “Rigby” by them.