What was the inflight entertainment in the early seventies?
During that time, the in-flight entertainment mostly available on flights was radio1 and/or TVs. Although it was not on all flights. Oops, that is true even today.

The radios may or may not have channels. If they had, there was a knob on the armrest, next to headset jacks, to switch channels. The headsets were pneumatic units like this:



The in-flight movies were shown using one or more common CRT TVs mounted on ceilings or walls and audio could be heard using the headsets. The first time a movie was shown on an international flight was in 1962 by PIA. It was more like the picture below but instead of LCD monitors, there were CRTs.



In mid-seventies, Braniff Airways was the first airline to offer video games in-flight.

By 1979, electronic headsets replaced their pneumatic counterparts, which improved audio quality.



Wikipedia has a detailed article about the history of in-flight entertainment:


  In 1971, TRANSCOM developed the 8mm film cassette. Flight attendants
  could now change movies in-flight and add short subject programming.
  
  In the late 1970s and early 1980s, CRT-based projectors began to
  appear on newer widebody aircraft, such as the Boeing 767.


This website has nice slideshow about the history.



1: Not actually a radio, but recorded audio