This is Fischer 374 with the inscription: "Polskie Termopile". Because of this inscription, the stamp was not brought into circulation.
Termopile is the Polish word for Termopylae, a Greek-Persian battle, fought 480 B.C. The Poles called their defence of Westerplatte the "Polish Thermopylae".
Westerplatte is the promontory guarding the harbour entrance of Gdańsk.
Battle of Thermopylae