[John Vasco]

I thought the German word (and I may get the spelling wrong here) Stellvertreter identified the individual who took on the administrative duties when there was no permanent person in charge. The Namentliche Verlustmeldungen that I have seen confirm this.

So, Martin Lutz was Stellv. Gr. Kdr. of Erprobungsgruppe 210 when Rubensdörffer was KIA on 15th August 1940, and Werner Weymann was Stellv. Staka of 1./Erpr. Gr. 210 for a short time when Lutz stepped up to be Stellv. Gr. Kdr. again after the loss of von Boltenstern. Both were confirmed in the posts within a few days.

Or perhaps Wolfgang Schenck, who knew his shit inside out on the Luftwaffe, got it wrong in conversations/discussions with me...