Based on the .375 Winchester rifle cartridge, this custom round was meant to fit between the .357 and the .445, but is no longer produced. It had a tapered case that was prone to sticking in the cylinder after firing.
A super magnum is a longer and/or more powerful version of a “magnum” cartridge. Although the term “super magnum” typically a refers to a handgun cartridge, created by lengthening an existing straight-case design, it can also refer a rifle cartridge, such as the .17 Winchester Super Magnum (WSM). In this case, it simply denotes that is of greater power than existing “magnums” of similar caliber, similar to other designations such as “Remington Ultra Magnum“. The most widespread of these cartridges are the “SuperMag” family of super-magnum handgun cartridges that were proposed and tested by Elgin Gates in the 1970s.