[Bill Walker]

The number is probably believable, if you don't distinguish between RAF and RCAF. The RAF and RCAF were, in many ways, closely connected at the time. Anyone signing up in the RCAF stood a very good chance of serving with an RAF unit, especially early in the war.

Google "Clayton Knight Committee" for more information. This was the officially sanctioned organization that recruited skilled Americans in America for both the RAF and RCAF, with under-the-table approval by the US, Canadian and UK governments. This organization was started before Canada entered the war in September 1939, to handle the huge numbers of Americans looking to join the RCAF or RAF even then. They had signed up over 8,000 by Pearl Harbour, mostly with prior aviation experience or training of some sort. To this number, add many more that crossed into Canada or the UK on their own and signed up there. Most of these had no prior aviation experience, and quite a few were rejects from the Clayton Knight operation. The Clayton Knight people usually supplied rejects with detailed information on how and where to enter Canada, and who to apply to once you got there.