Thanks folks; that's what I reckoned.
To be honest, my initial thought was that the driver didn't see many of such tickets and therefore just wasn't sure of the validity. However, his subsequent claim to have specifically asked his supervisors about validity did make me stop and think. Subsequent journeys on other 57s didn't throw up any problems - in fact, on one of them the driver was receiving some kind of training instruction, and queried the ticket with the trainer, who confirmed validity but said it should be recorded using the "Explorer" button on the machine!
RE: Your Travel Queries