Unknown author of the successful attack.
 

Colleagues. I can't solve yet another mystery of the war. According to German documents, at 5:27 on December 9, 1941, south of Brest, a single British aircraft attacked four German auxiliary minesweepers and sank M 1203 with a direct bomb hit. This is the only British source that mentions this.
German fishing vessel BURGERMEISTER SMIDT (500grt) was sunk by British bombing at Brest.
http://www.naval-history.net/xDKWW2-4112-39DEC01.htm
And in the Admiralty War Diaries for December 9, 1941, there's an entry
British Air Activity. S S S S Attacked 5 merchant vessels, 47°34' N.
4°34' W . , 360°, 6. Result unknown.
(A/C. duty J., 0345/9 to 19 Croup.)
https://sccd.royalnavy.mod.uk/-/medi...3638267a3a015b
which suggests that the aircraft belonged to the 19 Coastal Command Group. But these weren't 10, 217, or 502 Squadrons. The difficulty is that no one has claimed responsibility for this attack, which is quite rare. I would be grateful for any help solving this mystery.