50 C. Special edition \"Congres you B. I. T. 1930\" with overprint in type I, overprint inverted, properly franked multiple franking on local letter Ghyvelde 5.7.1940 with on the back arrival stamp from 6.07.1940, faultless preservation, expertise Lothar Herbst VP (2019) : \"the preservation 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.-