There is no way to tell without pulling heads and measuring.

They were all over the place when new so I don't think there was an average.

This why you hear stories about certain cars running so hard back in the day.

In the town I lived in the 60s-70s there was a 1968 383 RR bought new there with an auto trans, 3.23 rear, and only non stock item was a set of headers hooked to the factory exhaust. People would come from several towns away to race this car with there 440 Magnum cars, Charger and Coronet R/Ts and GTXs. 9 times out of 10 he would beat them in the 1/4 mile street race. It was just one of those cars.

I would bet if it had been torn down and checked closely you would have found that the compression was at or above the factory stated rating and that the tolerences were closer to a blueprinted engine than the typical factory engine.

These engines were put togather quickly, by humans, and with whatever part came next out of the bin. It was a crap shoot and some times you got lucky and everything in a particular engine was just right.