This is a screen of cut marble that is a bit of “trompe l’œil” in that it tries to look like a screen of wood, complete with a lock and keys. A notice indicated that it would be closed (with a marble door on hinges) when a religious council was held. It is halfway down the right hand side of the floor which leads to the mosaics there.

Elsewhere I read it is certainly not an original part of the church but a later addition. I took some pictures of details.