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This is probably the lid for the vent to the burial chamber (crypt) under the cover of Pope Pius XII
Pius XII (1876-1958). The 200th pope who served during World War II. Opinions are divided about his activities during the war, with his supporters saying he saved Jews, and his opponents saying he did not condemn the extermination of the Jews.
Translation of the text on the sign: PIO XII - Pius XII PONT MAX - High priest ANNO DOMINI MCMXL - 1940 in the year of the Lord PONTIFIC II - The 2nd bishop