From 1902 to 1907, certain postcards had to be franked at the foreign letter rate. The picture postcard shown above illustrates this fact. I doubt that many postal historians could correctly explain the rationale for the regulation.
The postcard was sent on August 28,1905, from Luxembourg-Ville to Havelange, Belgium. It was franked with a pair of the five-centime Adolphe definitive correctly paying the ten-centime UPU postcard rate. But nonetheless it was taxed 20 centimes on August 29th upon arrival in Havelange. Why?
The explanation is quite simple. This postcard has a divided back, with the right side reserved for the address and the left side for a message. Such divided-back postcards were invented in 1901 by a Germany printer named Hartmann. Before the advent of divided-back postcards, so-called undivided-back postcards reserved an entire side of the postcard for the address with the message restricted to the other side.
Divided-back postcards began appearing in 1902 in England, in 1903 in France, in 1904 in Norway and Russia, in 1905 in Sweden and Germany, and as you can see from the example above, in Luxembourg as well. Within domestic mails, they were accepted just like normal (undivided-back) postcards. However, under the Universal Postal Union (UPU) regulations, they were not allowed as international postcards at the postcard rate. Instead, these divided-back postcards were charged the foreign letter rate until new regulations were adopted by the 1906 Rome UPU Congress effective October 1, 1907, that treated divided-back and undivided-back postcards equally.
The letter rate from Luxembourg to Belgium in 1905 was 20 centimes. Thus, this divided-back postcard was correctly handstamped "T" [taxe] and marked "2" in blue crayon [for two gold centimes], indicating the ten-centime letter-rate deficiency, which was doubled and collected from the recipient at Havelange! (Prior to October 1, 1907, the deficiency rather than the amount due was indicated.)
Thanks to the Internet, these days it's easy to acquire scarce Luxembourg stamps and covers if your bank account is sufficiently large. But you can't buy the philatelic knowledge required to appreciate postal history gems like this postcard. That takes dedicated research!
Who has a divided-back postcard posted from Luxembourg during the period from 1902 to October 1, 1907, that is properly franked at the foreign letter rate to the country of destination? I'm still looking for that gem!
Here's another example!
Divided back postcard sent from Luxembourg-Ville, 7 Jul 1905, franked at the 10c UPU rate for undivided back postcards. At the New York exchange office, 8 Aug 1905, taxed six US cents (= 30 centimes) = double the 15-centime deficiency, the UPU letter rate from Luxembourg to the United States then being 25 centimes. Troy, New York machine cancel, 9 Aug 1905, for delivery to Lansingburg, New York.
Ch. Bernhoeft Serie 1905
Luxembourg - Pont du Chateau et rochers du Bock.