I wasn't there in the 70s, but from what I understand potency of the 70s wasn't nearly what it is today. I have no source for this, but I've been in countless circles with long time smokers who all correlate the same story; that modern day mids was literally the best quality Marijuana you could find in the 70s. So that's probably about 10% THC compared to 30% THC which is not uncommon today.