As pjaln says, this rifle was originally firing the 400/350 RIMMED cartridge.
Not to be confused with the later introduced 400/350 Rimless Magnum which also utilized a magnum length Oberndorf Mauser action but not the slant box stepped ring version as this rifle has.

As for the possibility that the 9.3X74R cartidge may not fit in a Siamese Mauser... I am not sure. I figure that if the 416 Rigby can be shoehorned into a standard length Mauser, maybe it could fit.