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On the sign:
PIO IX PONT MAX ANNO DOMINI MDCCCLVII PONTIFIC XII
This is probably the lid for the vent to the burial chamber (crypt) under the cover of Pope Pius IX The year 1857 is the year of the birth of Pope Pius XI and not of Pius IX.
Pius IX (1792-1878), the longest-serving pope (31 years)
Translation of the text on the sign: PIO IX - Pius IX PONT MAX - High priest ANNO DOMINI MDCCCLVII - 1857 in the year of the Lord PONTIFIC XII - The 12th bishop