50 C. Special stamp "Kongress du B. I. T.", horizontal pair, with hand-stamped overprint in Type I, used as a valid multiple franking on a letter from "Ghyvelde Nord 4-7-40", to Dunkerque Nord with a reverse arrival stamp dated 5-7-40, flawless condition, certified by Calves, Dreyfus, Roumet and expertise by Lothar Herbst VP (2025) "The condition is impeccable. The stamps are known in very small quantities, therefore they are very rare."