They're referring to none of the above. They're speaking of the .400/.350 Nitro Express in slant box Mausers. The .400/.350 was sometimes chambered in double rifles, but was primarily used in slant box Mausers and falling block single shots. The rimless .350 Magnum came along later and was somewhat popular, but never as much so as the .400/.350. The .400/.350 used a 2 3/4" bottleneck case with a .35 caliber 310 grain bullet, with 43 grains Cordite for 2000 fps.