25 C. Mercury Head, horizontal "Occupation of Northern France" overprint type I, variety "Inverted overprint", block of four with shared overprint across all four values, correct franking on letter "Ghyvelde 07.07.1940" to Dunkerque, arrival 08.07.1940 on reverse, perfect condition, stamps known in small numbers, thus rare, very rare with inverted overprint, expertised Roumet, Ceres, Lothar Herbst VP expert opinion (2023) "Condition faultless.", Mi. 1.100.-