Sorry, the ‘60s-’80s toilets are 3.0 gal, not 3.5 (not sure if there are different regulations then for different places, but that is the parents’ volume). Thus is it less to flush once old than to flush 2x in the modern requirement.
I don't know where you are getting your data, because as a service plumber and plumbing contractor, I was around when the 3.5 gallon flushers were introduced, and I'm calling that about 1980.
It took until the late 1960s to make the complete switch over from 7 gallons to 5.5 gallons in manufacturing.