Month names collection / metastudy

[Koen G]

My best guess is that the writer more carefully wrote out each letter (for some reason) and used "capital" letters.

The "rule" appears to be that common short words like "in" use regular initial-i, and others get initial-j. For example I've seen a couple of instances of the possessive pronoun "jhren". Also very frequently the abbreviation for "junkfrau". But like you say, it mostly ends up being used in common names 

About the careful writing idea, I'm not sure if that holds since in this word, "e" and "r" look more alike than they should. In careful writing, I would expect each to adhere more to its most recognizable form. But maybe some uncertainty is indeed at play, like a scribal stutter. Maybe some tension between spoken dialect and the norm, but that's pure speculation. A very interesting find either way!