When my research unearthed a will in which the lady left her “Mattins of Notre Dame” to her daughter, I had pause to halt. I’m not well versed in such matters, and had no idea what, exactly, a Mattins of Notre Dame was. I did know, of course “….the canonical hours of Matins (after midnight), Lauds (before dawn), Prime (at daybreak), Terce (mid-morning), Sext (midday), None (afternoon), Vespers (sunset) and Compline (before bed)….” But Matins could hardly be left to someone in a will! So what was the item in question?
Poking around on Google led me to this British Library site . As the link tells you, it concerns books, mainly medieval prayer-books. Books of Hours.
So was the Mattins of Notre Dame simply a Book of Hours? Or was it a very specific Book of Hours? If anyone knows the answer, please say!
murreyandblue 
There is a [Book of] Hours of Notre Dame, made in Bruges around 1470. It’s available on the World Digital Library for download as a PDF here:
https://www.wdl.org/en/item/251/
The download link is below the illuminated manuscript page.
Regarding mattins, it seems to be a British variation on Matins. Perhaps the lady left a book containing only the Matins portion of the Hours of Notre Dame?
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Following your bread crumbs, I found this. So, i would say, it does seem it is a book of hours. But the specific name drew to an artist, Jaquemart de Hesdin, a Flemish painter (1355-1414). I’m sure you came across this, but your blog intrigued me to look.
https://www.1st-art-gallery.com/Jacquemart-De-Hesdin/Hours-Of-Notre-Dame-Matins-The-Annunciation-From-The-Tres-Riches-Heures-Du-Duc-De-Berry.html
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