The case dimensions are significantly different than the 577/500 no 2, rim dia and base of the case. Also the bullet is heavier and larger in dia. The shoulder is much sharper as well.

As someone else posted, this I believe is the elusive rimmed version of the 12.7x70 Schuler ie the 12.7x70R.

I have a set of custom dies for the cartridge. The cases are made from shortened 577NE.