De Grunne was in fact the top scoring, non-Spanish ace in the SCW. He flew a Fiat CR32 . Before that he fought as an infantryman in the Falange, and it was only after he was wounded and in hospital that they found out he was a pilot.
As already mentioned he flew a Hurricane in the Belgian Air Force in 1940, but joined the RAF later.
There's a hint to some cloak -and dagger story in Portugal in 1941.