Nitro,
I've made a few inquiries about the 7x75R. Saw a break action single shot at the SCI show this year made by Philipp Oliendorff. The rifle was outstanding but I didn't know much about the cartridge. I learned it is the Vom Hoffe cartridge, and suspect it can be made by necking 9x74R brass down to 7mm. Sounds interesting, but not sure what performance would be gained over something like the 7x65 or necking down the 8x68 to 7mm.

Here's a picture of the rifle