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On the sign:
ZNAK POMIAROWY G.U.P..K USZKODZENIE POOLEGA KARZE AD 0360
A landmark made by GUPK, the National Measurement that was in Poland between 1945-1952, then changed to the Office of Geodesy and Cartography - GUGiK
G.U.P..K - Główny Urząd Pomiarów Kraju - National Measurement Office
Photographer’s Note: Not a manhole cover, but something similar. In the middle of a Forrest, I think they used it for surveying or distance measurements.
Translation of the text on the sign: MEASUREMENT MARK G.U.P.K DAMAGE IS PENALTY AD 0360