I read through this thread and don't see that anybody mentioned Eastern's Lesiure Class.

I was something like 12 years old at the time (some 35 years ago). When you bought a leisure class ticket (no doubt at a travel agency), you were essentially agreeing to fly standby . As I remember, the fare was substantially cheaper, but you took the risk of having to wait for space to get to your destination.

I sort of remember getting stuck at FLL trying to get back to LGA after Christmas.

I believe that Leisure Class was an Eastern only phenomena