At page 63 of the Basien-Hoffkamp rate book, we see several obscure special Luxembourg postal rates that were in effect during the 1940 to 1944 World War II occupation. And thanks to the work of Dieter and Fernand, we know that these include a 20-Rpf., 20-g letter rate to Hungary and Slovakia. But when I was last in Luxembourg, one of the local collectors remarked: "We never see uses of any of those rates." Well, wow! Here's one. It documents the 20-Rpf. letter rate to Slovakia.
This remarkable 50-Rpf. registered cover emanated from Differdange, September 1, 1941 [Differdingen [b] cds, transit tourist cancel Luxemburg [e]]. It is addressed to 'Poste restante, Kasihan (Slovakei)' and was censored in Vienna [censor tape 'g']. Apparently unclaimed, it was returned to Luxembourg endorsed Zurück, unzulässig! The Differdingen [c] backstamp dated September 8, 1941 documents its return.
The 50-Rpf. sole franking correctly pays the 20-Rpf. letter rate and the 30-Rpf. registry fee. Absolutely precious!