Gundog, you are right on the two points you mention.
Rigby did in fact commission Mauser to produce the first magnum length actions for the 400/350 rimmed cartridge.
These actions had a bit of a slanted magazine which tapered toward the front for the rimmed cartridges and had what is widely referred to as a "step ring" magnum action.
The step is a reduction in the diameter of the receiver ring where the barrel threads in. I don't know why they did that though.
The 400/350 used a bullet of 310grs and the 400/350No.2 which had the exact same case used a pointed bullet of 215grs at a higher velocity.

The later 350 Rigby Magnum rimless cartridge was, I believe, only offered with a magnum length action with the fully cylindrical receiver ring.