That is not an umlaut. In old German script to differentiate between an "n" and a "u", a hooked line was added over the "u" in a lot of hand written material.
As I am currently going through pages and pages of old German ancestry notes and graphs, that little hooked line at least lets me know where the vowel is amongst all of the up and down strokes!
Thanks very much for this!
I can appreciate your challenges with old German manuscripts.
They are usually too hard for me, especially when we have long chunks written in the Sütterlin style.
I have seen the Sütterlin "u" before but I had completely forgotten about it.
I must admit I usually stick to printed German and even then I try to avoid any books in a Fraktur/blackletter font.
Here's a snip from Wikipedia where we have set out in a column these three pairs of upper and lowercase German letters in their Sütterlin forms: