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.
See additional covers in the basilica Click for cover's details
, Click for cover's details Translation of the text on the cover
PIO IX - Pius IX
PONT MAX - High priest
ANNO DOMINI MDCCCLVII - 1857 in the year of the Lord
PONTIFIC XII - The 12th bishop