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.

Also, what of the many modern rifles stamped "275 Rigby" - do you need to have ammunition headstamped the same to get through Customs in Africa? I've heard that is the case but don't know for sure.