The stamps are originally from series issued by Greece between 1935 and 1939 featuring themes from Greek mythology and history and social welfare. The overprint was applied during the Italian occupation of the island of Paxos (Paxo in Italian) and Kythira (Cerigo in Italian)around 1941 and 1943. Many stamps from this series also had a fascist motto overprinted on the reverse side, such as "THE AXIS VICTORY IS THE VICTORY OF EUROPE" (in Italian or Greek). These particular stamps with the "Isola Italiana di Paxo or Cerigo” overprint are generally considered to be "Cinderella stamps" or unofficial local issues, meaning they were likely not authorized by a central postal administration for general use. They are philatelic items collected for their historical context,they are considered fascinating by collectors and can be difficult to find. Market prices for such stamps can vary significantly based on condition (mint hinged, never hinged, used) and demand on various platforms. The stamps are in excellent MNH condition, see pictures for your own impression.