This is an enveloppe “Sammlermarke Bezogen von der Versandstelle für Sammlermarken Krakau” (collectors stamps from the sending post office for stamps in Krakau). This fell under the Deutsche Post im Osten, in the “Generalgouvernement”.
The enveloppe contains two local DRK postcards with first day stamps: from Krakau and from Warchau.
While the cards are not that hard to find on the collectorsmarket, the enveloppe is extremely rare and only the first i’ve ever encountered!
A very rare DRK collectable!