I would say 100% that is a standard M98 action that has been opened up for the 416 Rigby cartridge. You can see the amount of opening up done on the feed ramp/lower locking lug recess area. I am not sure but I imagine the true Magnum Mauser action also had an extended length magazine follower not the standard version that leaves a gap in the front as the one in the photo does.

My own Type A standard length Mauser has been opened up for the 404 Jeffery cartridge and the magazine and feed ramp look very similar to that in the photo. The feed ramp for the 416 is even more radically ground away to fit the longer again 416 Rigby cartridge.