Lotnicza overprints (see Fischer 342 and Fischer III)
Lotnicza = Airmail.
Piast was the legendary founder of the Polish royal line. Piast was the chief of the Polonians, Slavic tribes who settled on the Warta river banks near present-day Poznań. He united the surrounding tribes into a single political unit. The term Piast was later used for all Polish (as opposed to foreign) candidates for the Polish throne. During the years 966-1370, the Polonians were ruled by the Piast dynasty.
Find other Polish stamps depicting the Polish eagle