A post on
http://www.valka.cz/newdesign/v900/clanek_10330.html (in Czech)
has a biography about pilot Ladislav Svetlik.
It seems that Svetlik saw quite some activity over France in May 1940, including several victories.
The date 21.5.40 isn't mentioned as such, but his engagement that day may be described.
Perhaps a Czech reader may help us out?
Regards,
Leendert
Brugge/Belgium