This manuscript is very precisely dated, having been finished on 19 June 960. The names of its copyist, the miniaturist Florencio and the calligrapher Sancho, are recorded, and they left their portraits in duplicate alongside the large omega of the colophon. Little more than the name of Sancho is known, other than that he was a priest. His master, Florencio, has left us more details scattered here and there in six extant codices and seven donation charters which he drew up as the notary of the Counts of Castile. He is believed to have emigrated to the North from Arabic-dominated southern Spain, and is recognised as outstanding among Spanish scribes.
He copied the codices in the Mozarabic Monastery of Valeránica, on the banks of the River Arlanza. This was protected by the Torre de Ómar [Omar's Tower], Omar being another Christian refugee from the South. This fortification has left its name to the present-day village in the Province of Burgos, Tordómar. The Monastery of Valeránica did not survive beyond the tenth century, perhaps as a consequence of the Arabic raids led by Almanzor. It is not known for certain how the codex came to St. Isidore's in Leon. It may be conjectured that it was donated to this, their favourite, church by Ferdinand I and his wife Sancha (1037), who had previously been Count and Countess and King and Queen of Castile.