The first and main part (pages [1a]-154) of this manuscript contains a remarkable collection of pope Leo the Great’s (440-461) epistles. The content itself of this codex very much attracted the attention of his first editor, Pasquier Quesnel, in 1675, and then in the following centuries, above all in the first decades of the twentieth, also the attention of scholars concerned about the critical edition of the text. If the results of the long and somehow controversial researches quite commonly agree to identity the archetype of this manuscript in a collection compiled at the epoch of the Schism of the Three Chapters (sixth century) and used by its upholders, in primis the Aquileian Church, it is still to clarify which was the possible centre, interested in its dogmatic content, that copied it and added the dossier on the unjust excommunication contained in the last folia. The marginal notes in humanistic script spread throughout the manuscript however evidence its presence in Friuli throughout the fifteenth century: there is recognizable the hand of Giacomo da Udine (1410 ca. – 1482), canon of the Chapter of Aquileia, copyist, literate, friend of several humanists of his time, among whom Guarnerio d’Artegna himself.