Re: what was the parent case for the 338 RUM
You know, I don't know what the short mag parent round is. The 404 has a base size of 540 and the short mag is .556 so there is something different. They may not have a parent case after all. Winchester and Remington can make anything they like from scratch, just like they've done for a hundred years. I'd be surprised if it makes any difference to them what they start with when they make a new cartridge.
If you have something that you disassemble and reassemble enough times, sooner or later, you'll have two!