Between 1964 and 1968 my older brother and I (aged 8-12) regularly flew unaccompanied between UK and USA on BOAC, Pan Am and TWA. It was on a TWA flight (DC8 or 707) that we first saw a film with a projector hanging from the ceiling but we had no money so could not hire the sound tube earphones. Then Pan Am started showing films, but again we could not afford the earphones, incredibly unfair to us! On subsequent TWA flights the flight attendants used to take pity on us a slip us headphones without charging; I think they were quite expensive then. Pan Am never gave us any so we preferred TWA! BOAC started films later which was a shame, but at least we had the best flights available then....BOAC/Cunard Super VC 10s.
Schroedinger's SU Carb balancer remark got my interest as I had an MG in the late 1970s and bought a Gunson tuner for its twin SU Carbs and it instantly reminded of those old "ear trumpet" style earphones!