From what I've observed, a few slant box 400/350s were rechambered to the later 350 Magnum without altering the bolt face as there is only about .005" between them. I suppose replacing the extractor would have been part of a proper conversion though.
That would explain the odd step ring/slant box 350 Rimless Magnum one occasionally sees.

The 350 here:
https://www.proxibid.com/Oberndorf-Rigby-Mauser-Bolt-Action-Sporting-Rifle/lotInformation/47410040
to my eye, doesn't appear to be a slant box 350.
The only way I am able to tell them apart by side view photos is to look for the far forward edge of the floor plate.
The auction rifle has the forward edge pretty much straight down from the rear side of the receiver ring - as one would expect from a straight down magazine whereas XAUSA's Rigby has the forward edge well behind a vertical line drawn from the rear edge of the receiver ring - magazine slanting rearward from the action bottom.
As heartbreaking as the fire picture is, it is a very good visual of a slant box magnum Mauser barreled action.