Sometimes you find philatelic beauty where you would least expect it. Here the 35c surcharge on the double-surcharged 30c Écusson postal card appears in tandem with the 35c surcharge on the 40c Charlotte (1st series) definitive—certainly an unusual, if not unique, pairing! The postal card is further enhanced by another pair—two 10c+5c Princess Elisabeth 1927 Caritas semi-postals.
1 April 1928
90c UPU Postal Card Rate
Incidentally, have you noticed that the single-bar 35c/40c/30c postal card was used mostly in 1928, but the double-bar version seems not to have appeared until 1929. I’ve never seen a use in 1928. The Luxembourg Handbook gives the single-bar and double-bar surcharged cards only one number (89); Prifix is more savvy, assigning numbers 83 and 83a to the single-bar and double-bar surcharges, respectively. And I agree with Prifix that the double-bar surcharge is the scarcest. Unfortunately, recent editions of Prifix no longer list postal stationery. What a shame!