I think the important factor is "built by Oberndorf in the thirties". All of the stepped receiver ring, slant magazine magnum Mauser actions I have seen (and I have owned several) are pre World War I, as is the one on my rifle. In my opinion, Oberndorf magnum actions as we know them (and I own several of them also) appeared after World War I.

Rimless cartridges work with slant magazines. The usual conversion by Rigby of pre-War .400/.350's and .350 Magnums was a rebore and rechambering job to .375 H&H. Rimmed cartridges can also be made to work with straight magazines, as the P14 Enfield demonstrated. Mauser evidently saw the rimless belted cartridge as the wave of the future and acted accordingly.