The Modern Forgery Hypothesis

[Jorge_Stolfi]

Do you think one 15th c. scribe knew these forms and used them for some reason? Do you think multiple scribes were penning these names, each individual writing in their own language and style?

My preferred theory is that whoever wrote the month names was trying to write them in a language that he did not quite know.  Maybe he copied from some source, with some errors.  Maybe he knew how they sounded like but did not quite remember how to spell them.

That "whoever" may have been a German Scribe copying from a manuscript in "French" (in the broad sense, including Occitan etc.). 

Or he could have been the Author or a late owner who would have been a native speaker of anything but "French", but decided to write them in "French" because he intended to give/sell the book to a "French" person.

All the best, --stolfi