50 C. "Mercury Head" definitive, vertical pair from lower left sheet corner, sheet number "38149", "Occupied Territory Northern France" overprint type I, variety "Inverted overprint - top to bottom", correct franking on letter "Ghyvelde 14.07.1940" to Dunkerque, arrival 15.07.1940 on reverse, rare, expertised Calves, Dreyfuß, Roumet, Lothar Herbst VP expert opinion (2023): "Stamps faultless. Known in small numbers, thus rare, rarer with inverted overprint.", unlisted in Michel