This is Fischer 342 in typography (22,7 x 33,6 mm) and in a slightly different colour.
Goznak is a monopolist in producing Russian postage stamps.
The eagle is the coat of arms of Poland.
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
Coat of arms of Poland
Bolesław III Krzywousty
Old-catholic Mariavite Church - Cathedral in Płock