Numis letters are collectibles for coin collectors and philatelists . They consist of a letter franked with a special stamp with a first day or special cancellation and a special coin or medal that is related to the theme of the stamp and integrated either in the envelope or an inserted numismatic sheet . As a crossover product, it should appeal to collectors from different areas and thus reach a larger group of buyers. The philatelic sense of numiscover is controversial. Numis covers always make the most sense when the stamp and coin are not only thematically but also chronologically closely related and the motifs overlap, i.e. ideally were designed by the same graphic artist or engraver .