f17r

[Stefan Wirtz_2]

I think there's a misunderstanding here.

Yes it was.
So I understand the trouble here now.

What I meant to say is that "cz" in German manuscripts always (as far as I'm aware) stands for the sound "tz". Sometimes scribes write only "z", but the sound is the same. For example, in medieval documents you can find both czehn and zehn.

Not quite. In modern German, if first letter, the Z is standalone - it was different before, like your czehn and zehn. If somewhere midword, it is often the tz combination - today and in earlier times. 
In older manuscripts, the t may appear as c , I saw that yesterday in my own examples. But the letter can mostly be identified as T because it is a bit higher than the following "3" / Z. Not much, hardly recognizable. I have never seen a "real" cz with meaning "tz", so I asked for your example. 
This line in VMS f17 shows clearly a (real) c and the "3" / z, and these letters are the best visible of whole line. The "c" does not look like a medieval "t" here at all; therefore, each German reacts here with "cz is not one of our combinations, must be Czech!" as I did.

In summary: when the cz ligature is used in Medieval German texts, it stands for the sound "tz". But it is not the exclusive way to represent this sound.

Regarding the "c" as a shy litttle "t", this could be right.

What I asked you is the following: since you think we should look at other languages, do you know how the ligature functioned in medieval texts in those languages?

Late already, have to track that the next days.

But "Chronica" up there gives another hint: the "un" between "fünffhundert ein un dreyssig" with a line above it (just like in VMS) is an "und", where the dash serves as abbreviation for the "d" (or even "t", if a writer had other ideas of writing): I have seen this type of abbreviation in other german scripts before.
Must admit this would point a bit back to German as language of this VMS annotation, though the dash of f17 is spanning both letters; but when it is Czech or a different language, the dash may have a completely other function.