Between 22nd July and 12th August 1943. 22nd July was when he lost Bf109 F-4 10256 and 12th Aug was when 10132 was lost. There are pictures of 10132 returning from a sorties on his 50th Victory but no definite date.
regards
Mark
Ps 10132 survives in Canada.
http://109lair.hobbyvista.com/walkar...0132/10132.htm