Its a form of the 338 Edge Improved. It would be more accurately discribed as a 338-270 Allen Magnum. This is because the shoulder location is moved forward compared to the 338 Edge.
To be honest, the easiest way to form brass is to neck down a 375 RUM case just enough so that it will chamber with some slight resistance and then load with a standard 338 Edge load and shoot.
If you just neck up a 300 RUM case to 338 and load and shoot, the headspace will be loose. It will work but you may get more case wall stretching just ahead of the case head then you may want.
Shoulder angle, I have kept that a bit of a secret. Its steeper then 35 degrees, less then 40 degrees.....