Taking another look at this signature, it only occurred to me just now that Michelangelo signed this upside down. The conventional way to write this would have been with the letters "MICHAELA[N]GELUS BONAROTUS FLORENTIN[US] FACIEBA[T]" ("Michelangelo Buonarroti of Florence carved this") beginning on Mary's right shoulder and proceeding down. Instead, they begin at her abdomen.
There is an interesting story behind the signature: The work was commissioned by a French Cardinal for his tomb. The requirements was "to create the most beautiful work of marble in Rome, one that no living artist could better." Naturally, most established sculptors balked at this. Michelangelo was then a 22 year-old unknown. He accepted the challenge, and completed the statue in two years. When it was installed in the (since demolished) Chapel of Santa Petronilla at the Vatican, the story goes that huge crowds formed around it (there was no television or internet at that time to distract people from things that matter). Being a vain artist, Michelangelo visited these crowds from time to time, and supposedly heard people wondering which of the great establishment artists at that time must have sculpted it. Supposedly someone remarked that it must have been Cristoforo Solari, and this insulted Michelangelo. Most likely you have never heard of Solari. He was an excellent artist, but no Michelangelo. No one was, and no one ever will be.
So, Michelangelo broke into Chapel late that night and signed his name. Back to that signature: I never thought about this before until just now, but the reason why he signed it upside down is because the sash was already there on the completed statue, and he was left handed. Holding the chisel with his right hand and working from the top down would have been more difficult for him. Of course, he could have written the letters backwards. But he was in a hurry, and working at night. I don't think he gave the matter a lot of thought. Just my conjecture mind you.