Wherefore art thou, aberil?

[Koen G]

I also noticed the code-switching in several manuscripts: they were well aware of the Latin names, but especially in almanac-type manuscripts would include the vernacular names. 

I maintain that looking for the exact spelling variant (aberil) remains helpful though. Language change is not a personal choice. It starts by affecting one (often elite) group or region in society, and then spreads from there over a wider area. 

It's possible that the month name writer was in such an area that was in flux, in the middle of a transition. If they had used "p", this would be of little use since it's right there in the Latin "Aprilis". But the fact remains that they wrote the marked form (aprilis did not have "b"), so it remains useful to track where else people used that marked form.

It may well be that in their region, "b" and "p" had become indistinguishable in certain positions, so they freely switched. But once again, in that case it remains useful to track where this was done, because it's the marked form. 

The least we can say is that the writer was from an area which allowed b-use. Cross-reference that with attestations of marked forms for the other months, and you can hopefully hone in on a source.