Johannes,
Balthasar flew first with 3./StG 77.
He move with this Staffel to 2./Erpr. Gr. 210 upon its formation and, as you say, he flew his first mission with that unit on 15th August 1940 on the raid against Martlesham Heath.
On 5th October 1940 he was wounded in action over London and spent some months in hospital. Bordfunker throughout the Battle of Britain was Rolf Schilleng.
Upon discharge from hospital he returned to 2./Erpr. Gr. 210, and belly-landed Bf 110 E-1, S9+LK, W. Nr. 3485, in the evening of 21st March 1941. His Bordfunker was Uffz. Jachmann.
He transferred to a Aufklärungsgruppe 23, and flew recce missions in Russia.
He was then transferred to III./ZG 26, 8. Staffel as you say.
I met Balthasar during my attendances at the Wespentreffen in the 1980s.
Mrs. Aretz, me, and Balthasar at a Wespentreffen.
