Yes, I know they didn't say Fab - I was just pointing out that the word fab was trendy at the time it was made, swinging sixties and all that, and it was a sort of homage or reference to those times. There is no probably right about it, as you are talking to a Thunderbirds fan from way back then and it is definite - F.A.B. meant nothing.
However, as I have just found out, it was later given a meaning by fans. go to
http://uk.imdb.com/title/tt0057790/trivia and scroll down.