This stone sculpture Akt
(Eng. = Act) by August Zamoyski is from a 1992 souvenir sheet of four issued for Polska '93, the World Philatelic Exhibition held in Warsaw.
From Wikipedia, "Count August Zamoyski (1893-1970)...was an author of stone compositions in simplified and geometric form...In the 1920's he developed his own, monumental style in which he referred to Classicism. In his last period, Zamoyski was an author of expressive religious works.