The RAF Regt has always doubled up on numbers 27 as well for instance. I can guess that this is because they had no aircraft, unlike VGS or AAC.
Now were the numbers 21, 22 and 23 every allocated to the RSF Regt?
Pub quiz question. What was the lowest number that was never allocated as an RAF sqn number? Clue, it once appeared on the door to a sqn HQ in a film.