50 C. postage stamp "Mercury head", vertical pair with overprint "Occupied Northern France" in Type I, variety "inverted overprint - top to bottom", correct postage on a letter from "Ghyvelde 14. 07. 1940" to Dunkerque with a rear arrival postmark dated 15. 07. 1940, rare, examined by Brun, Pierre Mayer and expertise by Lothar Herbst VP (2023) "The condition is flawless. The stamps are known in very small quantities, therefore they are rare, with inverted overprint even rarer.", not listed by Michel.