I think I remember six cent stamps in 1973?
Six cents was 1968-1971. Of course, back then you always had leftover stamps from before the last rate increase, and you’d use one- and two-cent stamps as needed to get the right postage. Best thing the USPS ever did was to go to the forever stamps.
I was wrong... rates from 1863 till now: https://about.usps.com/who/profile/history/domestic-letter-rates-since-1863.htm