Lessee...75gr 4350 over a 300gr bullet...guessing a .375H&H. I'm very tempted to say Elmer Keith, but since mavracer beat me to it, I'll go with Jack O'Connor.
This must have some historical significance for you to be stringing us along like this. If it ends up being some competition shooter's rifle I have to say, I'm going to be rather let down. It's a beautiful rifle. But of what significance is it? No offense.
Just some of my old stuff. Frank built that rifle on a Pre-64 Winchester action for Mr. Keith. You can read about it in his books. He sent me the target & picture sometime in the early 70s. Thought you guys might enjoy trying to figure it out. The rifle is simply beautiful.