World War I Feldpost Message-Reply Card

 

K.D. Feldpoststation No. 1, June 30, 1915, addressed to Karl Hartmann, a German soldier, with auxiliary marks “Nicht zu ermitteln” [unknown], “Nicht” in a rectangle, and “Zurück” [return] cancelling the 10c Marie Adélaïde definitive.

 

Returned to the sender and charged postage due at the 5c domestic postcard rate at Luxembourg-Ville, 11 (?) July, 1915.

 

Where does this strange beast belong?  In a Marie Adélaïde collection, a World War I postal history collection, a Luxembourg postage due collection, or perhaps even a collection of privately printed postal stationery?  Or maybe just in a binder of curiosities?