5c Black & 10c Red
Luxembourg-Gare B, 18 Apr 1923, to Luxembourg-Ville VII, 19 Apr 1923
Luxembourg’s first postage due stamps appeared on January 15, 1907; the second issue did not appear until January of 1922.
Here the first issue 5-centime green-black postage due stamp (Prifix No. 1) was still being used in 1923—16 years later--along with a 10-centime green-red second issue due stamp, even though the second issue also contained a five-centime stamp.
The card, which contains a notice from Bureau des Postes de Luxembourg-Gare, was sent unfranked. The mixed postage due franking pays the 15c domestic postcard rate.
Here’s an example on incoming mail from Belgium.
5c Black & 25c Red
Tervueren, Belgium, 21 Aug 1922, to Rosport, 22 Aug 1922
(Underfranked 5c, but charged postage due at the 30c minimum, which was not abolished between Belgium and Luxembourg until 1923)