Eliae Vineti ... de logistica libri tres

The ancient Latins indeed noted numbers by their own letters. For instance, when they signified five by V, ten by X, and fifty by L. The Greeks, Hebrews, and other nations did so by their own. But the Sifras ciphers we mentioned are so neatly and skillfully invented according to the natural ratio of numbers, and so adapted for computing, that no one who learns them will fail to admire their beauty, embrace them, and prefer them to their own. Their Algorismus brought them to us across the Pyrenees from the Saracens who held Spain. Since those Saracens—a motley collection of Arabs, Moors, and other barbarians—used the Arabic language, some of our Logisticians have conjectured them to be the invention of the Arabs, others the Hebrews, others the Chaldeans, and others the Phoenicians. They rely on this argument: that all these nations use their letters perversely, as Pomponius Mela writes of the Egyptians, proceeding by writing from right to left, and that we observe that same method in establishing the places of these signs. For example, when the current year from the birth of Christ, the one thousand five hundred and sixty-fourth, is noted as 1564, we take 4 as the first among these signs, which is on the right; the second is 6, which precedes it; the third and fourth follow toward the left. I know indeed that philosophy flourished among those Arabs and Saracens in Syria before it did in Greece, and after Greece was subdued by barbarians, and after the Roman Empire was overthrown, so that I would not deny that they could have invented something like this, and indeed something more beautiful; however, when it comes to numbers...

Handwritten annotation in the left margin:
what letter this is?
5