I have owned and used magnums from 17 to 500, and I currently own and use few different magnum calibers.

My favorite is the first -- the NA&A 500/450 Magnum BPE 3-1/2"
I do not know of another magnum-named cartridge that precedes it.
In my Tolley it's a flat shooting ripper with the 350gr bullet.

Cheers
Tinker