In 1957, "premium grade" fuel had a Research Octane Number of 97. By the later 1960s, most name brands were closer to 100. Regular was about 93-95. Sub-Regular was 91 Research Octane Number. These are the reference numbers for the owner's manuals from up to about 1973. Past that time, when unleaded fuel came to be, and later catalytic converters, the Posted Octane Number was mentioned.
There are two methods of determining fuel octane. One is the "Research Method" and the other one is the "Motor Method". Research yields the highest anti-knock octane ratings. Motor, being a more severe test, yields lower anti-knock octane numbers. The "Posted Octane Number" on the pump (or "Pump Octane Number") is an average of the two methods. (R+M)/2. In general, PON+"4-5" yields Research Octane Number, if not exactly, pretty close. That makes the 93-94 PON equate to the prior 97-99 RON for Premium unleaded fuels. Add the lead back in, at about 4ppm (I believe) and it's back to 100 RON.
The "mid-grade" of 89PON would equate to the Regular gasoline of old times, on the lower part of the general scale of RON.
Back when we were doing good to find 91PON "premium unleaded), octane boosters were popular. There were some good ones that worked and others were just an alcohol additive of suspected improvements. None were really cost-effective for daily use, though.
Later, "premium unleaded" increased to 92, then 93PON. Back in 1974, in Lubbock, I could get Phillips66 FliteFuel Premium with a PON of . . . 95.5, which with a little lead still in it, should have been about 101 RON! But it, too, decreased as lead diminished and other factors became involved. It ran better in the '66 Newport, up there, than anything else, period. In DFW, it was the same as everything else, unfortunately.
I know some of this "consumer oriented" stuff can be a little confusing when cross-reefing it to a 1970 Owner's Manual fuel recommendation, but knowing WHICH numbers are being quoted is important.
CBODY67