There is a trend in the dating, but it doesn't cut off entirely. In fact, one of our top three tied entries has an "octembre" from as late as 1495. This is from Cambrai, which was aligned with the Low Countries at the time. Other highly ranked entries are also post-1450, and a bunch of them are "15th century".
It's good that we now know about this trend, but I would not build further speculation upon it yet.
More generally though, what is up with these marginalia writers? The one who wrote the German on You are not allowed to view links. Register or Login to view. seems to have been struggling with German. The one who wrote the month names apparently attempted to write standard French but seems to have been more familiar with a northern dialect. And we can't even decide if You are not allowed to view links. Register or Login to view. is a Romance or Germanic language!
It's good that we now know about this trend, but I would not build further speculation upon it yet.
More generally though, what is up with these marginalia writers? The one who wrote the German on You are not allowed to view links. Register or Login to view. seems to have been struggling with German. The one who wrote the month names apparently attempted to write standard French but seems to have been more familiar with a northern dialect. And we can't even decide if You are not allowed to view links. Register or Login to view. is a Romance or Germanic language!