Wherefore art thou, aberil?

[Aga Tentakulus]

We have already discussed this somewhere.
Based on the examples, one can cover the entire eastern Alpine region (Switzerland).
I am limited in my search across borders. However, I also see possibilities as far as the County of Gorizia.

If the spelling of month names can be roughly attributed to some region, is it possible to get a few samples of texts in this spelling variant from the same region?

Possible.
The most likely place is in the Diocese of Chur. Everything related to administration. Chur Archives.
According to the map:
Glarus GL (marked in blue) has already joined the Swiss Confederation. The others belong to the Grey League GR (Graubünden).
The white area (lower Calven) belonged to Tarasp and already speaks Bavarian (dialect) with Samnaun. Apart from the Rhaeto-Romanic.