I believe it was Mauser that made Barreled actions to Rigby, and I doubt mauser would deliver a 7x57 with a different twist so 175grain round nosed bullets could not be used. The sights ofcouse is regulated for the 140 grain but if the rifle are used on biggame the difference between the 140 and 175 at close range isn`t all that much to really matter.
Its funny in a way...When one are thinking: Rigby & .275, they are thinking of "Africa", but the rifle was made for use on the british isles. I have seen Rigby advertise their .275 as a stalking rifle for the scottish highlands...never seen them advertise a .275 for africa....at least not yet..haha