These early squadron numbers have a history going back to the post-First World War Canadian Air Force which disbanded in 1920.
Established in Canada in the 1930s most of these units were either disbanded or reassigned a 400 series number for overseas service.
Under Article XV of the BCATP, RCAF squadrons were attached to and/or formed within British RAF operational formations.
During the Second World War pilots who served in the 600-series RCAF squadrons were recruited from the Royal Canadian Artillery in England and Italy, and trained to fly at No.
Subsequent to the Second World War the RCAF expanded their squadron numbers to include 444–450.