Well John, I don't think you can say that GAY was a 99 Sqn code for I have a photograph of WJ332 with the GAY code and the 53 Sqn diamond on the tail. (I am trying to post the photograph but I can't get into the URL for some obscure reason).
I also have a note in my book about 53 Sqn which states that "after the Berlin Airlift, Hastings aircraft carried the last three letters of the radio callsign in large letters behind the roundel".
So, I had always figured that GAY broke down as:
M = British Military Aircraft
O = Transport Command
G = Hastings Type
A = Unit (In this case, Lyneham Pool)
Y = Individual Aircraft (WJ332).
The other letter used by the Hastings was "J" (For example, JAZ was TG564). You may also remember that the letters used by the Argosy were "F" and "K".
I'll post the photograph of WJ332 when I can sort out my problems.