Be aware that the self appointed AB Police will probably jump on this thread, as the quiz setter says that he doesn't want to see answers given on this site.
If I knew it, I would tell you just for badness but sorry it has me stumped too.
Have you missed the postal date? I have just had the results sheet. The answer is G - last letters in US phonetic alphabet used in WWII to 1956. Slightly remote I would have thought. No wonder fewer peolple are bothering with this part of the quiz.