Month names collection / metastudy

[davidma]

You mean something like this.
It's a dialect thing. Y with dots. Here, the i changes to ü in terms of tone. But also the double ii, as in (gsii / bii etc.)
Y with dots is also regionally the old (ü).

The sign on the y in the VM is not an umlaut, it is quite clearly a circumflex accent, so I feel it is much closer to the dotted y rather than the double dotted one. The months names also belong quite convincingly to the general french speaking world of the time rather than the german-speaking area.