I seem to recall there was a GAI / DIN a few years back (around the time that XIII Squadron reformed IIRC) that directed that the only squadrons that could include letters after their squadron number on badges (and I mean badges, for the use on flying suit
), letterheads etc were those officiclaly accredited with either being a BofB squadron (F) or an Army Co-operation Sqns (AC). The reasons given, I believe, were that in many instances the letters no longer reflected the true role of that particular squadron. This would not affect the official crest or standard though.
Perhaps someone with a bit more historical knowledge/memory may be able to shed more light on this?