Hello Chris and René,
Thanks for your compassion
and thanks to your help it now all makes sense to me:
It was Lt. Schreckenberger (III./NJG 1 or 3./NJG 2 according to his Personalakte) -
not Brandenberger nor Bogensperger - who claimed a Mosquito in the night of 12/13th February 1944 with a Me410 which the III./NJG 1 had some on their inventory.
Though his claim was not confirmed (and never was acknowledged at all) Schreckenberger was promptly invited to visit Gen. Beppo Schmidt (HQ I./Jagdkorps in Zeist/Holland) on 20th February 1944. He must have received orders to form a small Erprobungskommando (410) in Venlo, because Schreckenberger came to Venlo on 1th March 1944.
Next day he was joined by Oblt. Henseler who was transferred from the II./NJG 1 to Erprobungskommando Schreckenberger (according to the KTB II./NJG 1) or... 1./NJG 1 (says the KTB I./NJG 1).
This and other info from Henseler's flightlog suggests that the incomplete 1./NJG 1 acted also as Erprobungskommando 410, because its few Me410's had the Verbandskennzeichen of the 1./NJG 1.
There is still to do some more research but at least we now know how an unconfirmed claim with a Me410 against a Mosquito lead to the establishment of Erpr.Kdo. 410 if that ever was an officially existing Luftwaffe unit.
Thanks guys, much appreciated ! Warm greetings, Marcel