Treble One it is. The squadrons called eg Two One Six (216) are mostly ex RNAS squadrons whose original numbers were given a prefix of '2' when the RAF was formed.
The history of sqn numbers was once quite a 'structured' topic but things change and origins are forgotten. The RAF now has a policy on how squadron numbers will be dished out and nobody who recalls the origins is now around in a place of influence. However, the Canadians, Norwegians, Dutch and Belgians often retained the numbers their squadrons were given when associated with the RAF during the last unpleasantness between 1939 and 45. Hence 300 and 400 series units still in those air forces. Even the British Army's 600 series squadrons owe their origins, mostly to the RAF.
Inevitably, there are exceptions - hence 617, which survives despite the general 'low number' focus of RAF policy.
Old Duffer