Just to add some details on your parts from the 60s, Lego began switching from Cellulose Acetate to ABS plastic in 1963 and the switch was complete by 1966, so

Just to add some details on your parts from the 60s, Lego began switching from Cellulose Acetate to ABS plastic in 1963 and the switch was complete by 1966, so your estimation of part quality during that period seems to be spot on.