Ahh, the old days of G&A and Ross Seyfried boomer loads!
If my memory is correct, it was Dean Grinnell of Gun World. I am going to go ahead and admit it was 9 grains which of course you should absolutely positively never try.
All of the things mentioned by others do make a difference. Probably the things which cause excessive pressure more than anything else is brass thickness and seating depth.
Back in the 60s, Remington and Norma brass was very thick. Winchester was thinner.