50 C. Special edition "Congres you B. I. T. 1930" with overprint in type I, overprint inverted, correctly franked multiple franking on cover from Ghyvelde 14.7.1940 to Dunkerque with on the back arrival stamp from 15.07.1940, faultless condition, 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.-