Because the tête-bêche format is part of the attraction for collectors, titles published between 1974 and 1978, which contained two titles by one or two authors but which are not tête-bêche, are not regarded by some collectors as true Ace Doubles.
For more information about the history of these titles, see Ace Books, which includes a discussion of the serial numbering conventions used and an explanation of the letter-code system.