Yes it is my own invention.
I have plans on converting a 8,2x53R with the same idea. Maybe I’ll even pick up one of those nice ones that Igorrock mentioned…

I started by planning a cartridge that would need minimum changes to the rifle itself.
Since the measurements of my original wildcat were so close to .350 Rem Mag, I made mine match to find reloading equipment easier. I actually use a .350 Rem Mag sizing die and seating die I only made a new neck calibrator to the sizing die. I could even use .350 Rem Mag brass, but there are none available in Finland.

I got to test it a few weeks ago on a 300m range and it worked better than I expected.
The recoil without the silencer is quite uncomfortable but with it bearable. A 250grain Barnes TSX leaves the barrel at 770m/s and this is a safe load.