A nearly forgotten fact is that, before the early '70s there were 2 different fuel octane rating methods. I did a search using 'RON+MON' and found a site with a detailed explanation of the different octane rating methods. In the US, the Research Octane Number (RON) and the Motor Octane Number (MON) were combined (RON+MON) to give the Pump Octane Number (PON). That site also has a chart showing that the 91 RON octane rating is equivalent to an 87 PON rating.

Also, for 1972, cars sold in the US were required to be able to run on unleaded gas. Compression for the US was reduced by many car manufacturers as most unleaded gas at that time was availible only in regular grade.

On an engine designed for regular fuel, there are many factors that can cause pinging, including incorrect timing, lean fuel mixture, high engine temperatures, carbon build-up in the cylinders, engine modifications (shaved heads, different pistons) or a combination of any of these factors.

I have to run premium in my 60 220S and my '72 250 (with a higher compression '70 engine) but I used to run regular in my lower compression '72 220 without problems.

Happy Motoring, Mark

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DrDKW