Here is another one: a later musical addition to the margins of a 13th c. manuscript containing Petrus Riga, Aurura. It seems to be 16th c., latter half?
Apologies if it has been reported before – it seems to have been online a while.
It's quite a nice little 2-part setting, actually. I will attach an image of the page in question (image remains © Bibliothèque de l’Agglomération du Pays de Saint-Omer) as well as a transcription in score format for convenience (the text underlay might not be perfect in this transcription, though I tried to follow the placement in the source).