One of the first cartridges by Roy Weatherby was the .375 Weatherby Magnum, an improved version of the .375 H&H; Magnum. Weatherby's .375 was a good cartridge but it beat Holland's .375 by only about 100 fps. In 1953 Roy dropped the .375 and introduced the .378 Magnum. Despite nomenclature, bullet diameter remained the same. But the same cannot be said about velocity; the .378 pushes a 300 grain bullet along at close to 3000 fps. 

This text is based on information from “Cartridges of the World”, Hodgdon reloading manual, the cartridge designer and/or own resources.