It was not only an endorsement those days but you could do what they called a FIM (Flight Interruption Manifest). From my memory it was more than the discounted fares paid by the passengers. But it was the best way to get "rid" of any problems with the passengers as the receiving airline is happy that they are getting a better revenue than the discounted tickets they sell openly and the canceled airline is happy in a way the headache is also gone with having to book hotels, fight with the passengers and then have the regulatory body question them. Yes they have to dish out a bit of money on a canceled or a badly delayed flight because of the fare difference.