50 C. "Mercury Head" definitive, vertical pair, "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 Brun, Pierre Mayer, Lothar Herbst VP expert opinion (2023): "Condition faultless. Stamps known in small numbers, thus rare, rarer with inverted overprint.", unlisted in Michel