Hi,
Prien,
Jagdgeschwader 77, p.2437, notes that Riedel's claim was ~03.08.1944. On page 2113-2114 there is a lengthy first-hand account about the combat from Johann Twietmeyer, from a letter sent to Prien on 4 February 1990. Prien notes: "Datum des Einsatzes ungewiss, möglicherweise 3.8.1944".
Cheers,
Andrew A.
Air War Publications -
www.airwarpublications.com