This seemingly unspectacular cover, franked with the very attractive and scarce 3-franc United Europe stamp [Prifix 447] and posted from Rumelange, April 25, 1952, to Mettlach in the Saarland, arrived in today's mail from an eBay seller in Coventry in the UK. It illustrates an often overlooked special rate.
After WWII, the Saarland came under French Administration. And from 1949 to 1958, 20-g letter post from Luxembourg to the Saarland was charged the letter post rate to France (3 francs) rather than the higher, 4-franc letter post rate to Germany. So if you collect (as I do) sole frankings of this issue (or of other issues during this period), you'll want to include in your modern postal history collection a properly rated cover from Luxembourg to the Saarland.
The Saarland rejoined Germany on January 1, 1957, but the French franc remained the Saarland's currency, and the 3-franc letter post rate from Luxembourg remained in effect, for one more year, until January 1, 1958.
The cover shown below was posted on November 5, 1957, from Remich to Saarbrucken, shortly before the special 3-franc rate ended. It is nicely franked with the scarce 3-franc 1956 Europa stamp [Prifix 515]. The Remich tourist cancel reads "Le Soleil se Met en Bouteille ala Moselle" [The Sun on the Moselle is Put in the Bottle], reminding the collector of the fine Riesling wines produced by the Luxembourgian vintners along the Moselle.