Luxemburg, 3 Oct 1941
During the World War II occupation, letters and postcards from Luxembourg to Belgium were treated preferentially, with the Reich domestic 12 Rpf. letter rate and 6 Rpf. postcard rate applying. (In contrast, mail to neighboring France was charged the 25 Rpf. foreign letter and 15 Rpf. postcard rate.)
However, the special delivery fee to Belgium was 50 Rpf.—the fee charged for foreign mail, not the 40 Rpf. Reich domestic special delivery fee.
The commercial postcard shown above shows the Belgian preferential 6 Rpf. postcard rate and the non-preferential 50 Rpf. special delivery fee.
I also like the corner card:
[fine gloss paint]