In the late 60s and at least part of the 70s, the transcon airlines (United, TWA, American) had 10pm flights from Boston, JFK, and maybe Philadelphia and/or Friendship. On these flights, you could fly F for the price of daytime Y, and Y was discounted (not sure how much, but recall about 20%).

At that time, and this was when the government set the fares for everything except intra-state routes, there was 1 price for flying tourist (Y), and no such thing as discount fares or non-refundable tickets.