I do not know about how PADI was behaving in the seventies, but when I made my first OW course with CMAS, in 1975, the course was lasting 6 months, and did include training with CC rebreathers (ARO, pure oxygen, but just in the pool), twin air tanks with double complete regulators, and full deco theory with multiple stops (using US Navy tables). There were no specialties, and at the end of the course each student was evaluated and given either a one-star or a two-stars (AOW) certification. I got the two-stars, which did enable me diving down to 30m, with a buddy, within NDL.I recently heard a long time PADI CD say that through the 1970's, all agencies firmly believed that everything you really needed to know about scuba was in the OW class as it was taught by all agencies. There was no real need to teach anything more. This was all before tech diving and all the other stuff we know about now.
The following year I made the "second level" course (3 stars), lasting "just" 3 months, which enabled me to dive CC pure-oxygen rebreathers down to 10m and air tanks down to 50m, with deco (not exceeding the 9-meters stop) and with more than one dive per day (which was considered far more dangerous than deco, also requiring the use of the very complex tables for repeated dives). This was fully recreational at the time.
Tech was considered below 50m and with deco starting at 12m stop or more.
CMAS was not providing training for tech, you had to go to a professional school for commercial tech divers, such as Istituto Rossi in Vicenza, etc.. In those institutes, the tech course was one full year!