In the 1960s, Regular was 94 octane, and premium was 100. A few companies sold a mid grade at about 97.
There weren’t any Sunoco stations around my part of the world, but my recollection was they had a ‘pick your blend’ pump with 5 or so choices.
Indeed, you're pretty much correct on that. Sunoco (Sun Oil then) patented the blending valve located in the pump used to create the gasoline output with different octane numbers. The filling station had 2 storage tanks, 81 and 105 octane rating as I recall. There was a dial on the pump and you selected 81, 85, 89, 93 or 105 octane IIRC.
One of my uncles (a mechanical engineer out of Oklahoma A&M) designed the mixing valve. After WWII, he finished his degree at OK A&M then worked for Sun Oil until retiring in the mid-70’s. After the Normandy breakout, he was a replacement paratrooper into the 101st Airborne. Fought in the Battle of the Bulge through VE Day.