I feel your pain, often on a daily basis (try I.D.ing a slide whose label has fallen off... so much fun!)

However, keep in mind that even this late, books were gifted and re-gifted as lavish presents. It's not impossible that Marguerite may have gifted the book to Mary at some point, or loaned it to her for a time. My own quick-skim research hasn't come up with a reliable provenance, and the on-line catalogues are being intractable (in French... Coincidence?) so i can't try to find more about it.

Of course, I broke my own cardinal rule of researching manuscripts by running my first search on just Brussels Manuscript. ::sigh:: Always always include the number, Alix. Silly child.