Been some posts about that credit on BoB thread in Mil Pilots. Have seen some reports elsewhere of a stunt pilot called Milton Rubenfeld who joined at outbreak of war. Says he flew hurricanes in a small (4 - 5 aircraft) unit in spotting role for 420 sqn. But can only find mention of 420 as RCAF bomber outfit. Perhaps he got the number wrong?
Generally accepted reason for the credit is down to one of the film's producers, Benjamin Fisz, who flew Hurricanes with 303 sqn but did not start operations until October 40. Well, that's close enough?