50 C. Special edition "Congres you B. I. T. 1930" with overprint in type I, overprint inverted, properly franked multiple franking on cover from Ghyvelde 14.7.1940 to Dunkerque with on the back arrival cancel from 15.07.1940, very fine, expertise Lothar Herbst VP (2019) : "the condition is perfect. The stamps are only in very small number of pieces known, therefore are they very rare. Multiple franking with special stamps and inverted overprint are especially rare. ", Michel 2.000.-