This small (29mm x 20mm), late 1930's Polish dues stamp features maps of 2 Baltic cities, neither of which was in Poland at the time. The left map shows Szczecin (Stettin, Germany) and right map features Gdansk (Free City of Danzig), which Poland claimed.
The stamp is of the Liga Morska, the Maritime and Colonial League, which began as a public society to promote seafaring and colonial expansion for Poland. The League was quickly taken over by the government for its usefulness in promotion and propaganda,
From Wikipedia, "The Maritime and Colonial League was a mass Polish social organization, created in 1930. In the late 1930's it was directed by general Mariusz Zaruski and its purpose was to educate the Polish nation about maritime issues. It also actively supported the development of both a merchant fleet and a large navy, as well as colonies and overseas possessions." Some of Poland's prospective colonies included Brazil, Liberia, Madagascar, Mozambique, and Peru.
Before the German and Soviet invasions of 1939, the League had approximately one million members.