I found on the net the following story:
In England Fred Hillis was assigned to the 9th Air Force in France to a Republic P-47 "Thunderbolt" fighter group. When the war ended, there were five emblems designating five aerial victories painted on the cockpit canopy of Fred's P-47: four German swastikas and one American flag, so that meant he was an “Ace” ... he had shot down five aircraft in combat - four German planes and one American plane [a P-47 flown by a German pilot].
See the rest:
http://www.stormbirds.com/hillis/tboltkill.htm
Is there any historical truth to this story?
Dénes