I'd like to know how an operator cued these machines up for playback to air. You can't see a picture unless the machine is playing, so you can't do it in the way that you'd do with 1″C or cassette based machines (visual search). And to do it by the audio track, would be relying on someone else to have managed to put cue tones into the right spot, who wouldn't have been able to see the picture while they did that, either. Sure when recording a live program, you could start with a countdown and vision switch at the right moment to pictures. But I've seen programs where it's quite obvious that they've gone and done an edit to put in a countdown, and I'm darned if I know how they managed that.
If you have any quad tapes with old recordings on them, check that you do not have programs that have gone missing from circulation. Many programs, such as Doctor Who, Dad's Army, and many more, had many of their master tapes wiped, many years ago, due to lack of foresight, and their owners desperately want any copies back. Not to mention the program's fans want to see this programs even more desperately. For instance Doctor Who is missing many of their programs from before the 1970s. Please look through your reel titles, and contact their home bases (the BBC, etc.) if you find anything. They're far more concerned into restoring their program collections than giving anybody any grief about holding onto tapes they mayn't have authorisation for.