I had some problems with this piece at the very beginning : since it has no mensuration sign, I assumed first a binary time signature... with no luck! The tenor is then too short, and unavoidably clashes with the other part. Thus the piece must be read in tempus perfectum in order to make the parts sound well together.

However, things are not so simple, as one may learn from the comments sent to me by Olga Bluteau, and that I'll now give with her kind permission.

A first surprising point: indeed, the music sounds binary!

The cantus is written mostly with a C cle on the first line, but for a while it uses G cle, and thus some upper F can be written without using any ledger line. It goes as low as B flat under the C cle, and this means an ambitus (range) of a twelfth! That's unusual for a vocal part in this time period, specially for a cantus part; we'd more likely come upon such a range in a contratenor, often written as a "padding" part.

The tenor is notated with a C cle on the fifth line, instead of the fourth usual one, probably in order to let it go as low as B flat. However its range remains rather standard, since it doesn't go upper than the C shown by the cle.
The C cle on the fifth line was seldom used in this time already, and nearly disappeared as soon as the beginning of the sixteenth century; it was then replaced by the F cle on the third line, known as the baryton cle.

Notes and text on the facsimile are wonderfully readable! I have, however, no experience in reading early French... but it's a pleasure for me to reproduce now a reliable version of the text sent by Olga (she's kept the original writing, which looks sometimes strange even to a modern French reader - notice the absence of accents, for instance)
Native English speakers will forgive my attempt of a translation which, at least, will give them the rough meaning of these words:
Nivelle détail

Se je demeure despourveue
Et de douleur tant abatue
C'est ma dolente destinee
Qui ma a ceste loy menee
Qua jamais suis de tous biens nue

Car sur ma foy de dueil me tue
Quant ie pense que fus tenue
La plus eureuse qui fust nee
Et je me voy si fortunee
Que ceste grace jay perdue

Se je demeure [...]
Destitute I remain
And so worn out with pain
Such is my mournful fate
To hear a law I hate:
No good will ever gain

I feel with grief dying
Hopeful in my morning
Now sad in that evening
Full of despair I am
Of thou, sweet grace, lost gem.

Destitute I remain [...]

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