I found another "poxleber"

[npcompl33t]

In short, there are some words like wax, where the "x" is seen as the standard spelling. But replacing "ks" with "x" elsewhere is generally not done. Freedom of spelling in the Middle Ages doesn't mean that there were no customs, and randomly introducing x was not customary in MHG.

Note that what happens in the VM is even one step further, because you're not just replacing part of the root word with "x", you are including the genitive -s. This would be as if in English you'd write "Rick's book" as "Rix book". Or "the bank's money" as "the banx money". 

If you can find cases where genitive "cks" is written with "x", those would be more relevant. When I looked for them many years ago, I found only one. A shipment of bacon (speck) where the genitive form was written as "spex". 

This makes sense, and it removes most of the instances of x I found. A more targeted search did turn up a few examples under this criteria: 

1. spex = specks
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• Des ſpex nym ſouil du der ſalben machen wilt
  

the same recipe uses all three spellings: ſpeck for the ingredient; Des ſpex in the instruction above; dᵉs ſpecks later in the recipe.

2. volx = Volks
Cologne chronicle text, ReF F159. You are not allowed to view links. Register or Login to view.

• Want luzzel volx myt mu͗den ledín
  

a partitive genitive, volk + s → volx.

3. Koninx
The Cologne charter collection also contains:

• des vuͦrgenuͦmdes … Koninx
  

This supports x occurring in a genitive form, but it is a different consonant cluster than bock/speck.

About the mich with superscript "l", that's an interesting one - I guess you caught someone misspelling "milch" and then correcting it?

That would be my guess, its a one-time occurrence vs multiple other instances of milch in the same document, so definitely not evidence of widespread use. I thought it was interesting because of its proximity to the psuedo-latin charms. 


My search turned up another instance of likely misspelling producing a separate marginalia word. The You are not allowed to view links. Register or Login to view. transcription at 85rb,25 has: "wan de marix der vrouwen vp sticht", which seems to be a misspelling of matrix, as the same immediate passage references the womb and uses 'matrix' at 85rb,21 and 85rb,32. 

However it is not a particularly satisfying explanation to assume the author made multiple spelling errors in a short span of text.