Interesting question, never thought about it. Straight wall case has meant a rimmed cartridge to me. The .458x2, .458 Winchester, and .458 Lott, are pretty much what you're talking about, and are commonly referred to as "belted magnums". However, cartridge nomenclature doesn't have a lot of hard fast rules. If you want the belt to be just a different rim style it works. If you come up with something new call it what you want.