[ChristianK]

Thank you both for the quick reply. And of course, Staffel not Gruppe. You are right, Nick.
And in regard to the "L": I figured this might have been one the cases which occured in the last days of war, when older tactical markings were carried on with their planes to new units without a change. But all these Hs 126 are marked pretty consistently and not like a rag-tag bunch assembled from various sources. So I guess they could very well have belonged to a "real" 3. Staffel of another unit.

But to be honest, the number of possible contenders is low. The only other He 126 user besides NSGr. 7 in southeastern Europe at this time was NAGr.12, and there also just its 2. Staffel...

Guess this remains a mystery for now.