Florentine curiosity: a strange sculpture on the facade of the Palazzo Vecchio

If you enter in the beautiful courtyard of the Palazzo Vecchio built by Michelozzo and decorated in 1565 by Giorgio Vasari for the wedding of Francesco de ‘ Medici and Johanna of Austria, you can not refrain from observing, to the right of the entrance door of a stone carved with a profile right next to the statues of Philemon, made by Bandinelli and his wife, Baucis, sculpted by Vincenzo de ‘ Rossi.
It is one of many blocks of stone from the quarries probably stronger of the Boboli hill or from the Costa San Giorgio, but who was to make the portrait?
Perhaps was the famous artist Michelangelo during a discussion claimed to be able  to sculpt a rock without looking and with his hands behind his back;  the challenge was accepted and in a few moment jumped out a chisel and hammer.
Michelangelo went to work under the eyes of everyone, probably even the Palace Guards who were careful not to interrupt the work of the famous artist, and who could be allowed if not to him for damage to the facade of the Palazzo Vecchio?
But the question arose of what or who do not retract and rivalry, Michelangelo took as a reference the face of a poor man that the guards wore to the prison.
In a short time the portrait was finished under the eyes of all, and although it is not exactly a masterpiece, it must be recognized that to be done without looking and with his hands behind his back is not so bad.