Above what is now the entrance to the Bookshop, and which, as the inscription reminds us, was the entrance to the Museum in 1891, stands a large bas-relief in pietra serena sandstone. The rich and intricate decoration consists of several elements. On either side, two plaques display the coat of arms of the Opera del Duomo, featuring the Agnus Dei with the trigram "OPA." In the lower central section, a fragment of a Latin inscription can be seen, while higher up, within a Serlian frame, a large shield is prominently engraved with the Opera’s trigram.

The shield is adorned with scrollwork, surmounted by three feathers, and flanked at the top and bottom by diamonds. Ribbons and volutes intertwine around it, while at the bottom, two capricorn heads with a star on their foreheads emerge. Finally, on either side, there are two pairs of flame-bearing urns.

However, this is not the relief’s original appearance: we know that in 1843, the Medici family's heraldic balls were chiseled away from the shield and replaced with the Opera’s trigram.

Stylistic analysis further confirms that the relief was created between 1567 and 1569, during the reign of Grand Duke Cosimo I, as a testament to the Opera’s submission to the new government. Several symbolic elements of Cosimo I remain distinguishable, such as the diamond and feathers, the capricorn of his personal emblem, and the motto he shared with his wife, Eleonora of Toledo: Videbunt iusti et laetabuntur (from the Book of Job, 22:19).