Originally Posted by Sykes

Sort of. Your explanation is basically correct, but one minor detail is off. Travel agents were issued generic ticket stock that had 10-digit stock control numbers. Then they were issued a plate for each airline on which they were authorized to issue tickets which had a 3 digit number on it (e.g. 016 for United). The agent would then use an imprinter (like an old credit card imprinter) to print the plate in front of the stock control number to issue the ticket, thus creating the full ticket number. Thus they had one plate for each airline, but just a generic ticket book.