Interchangeable ch/sh and k/t characters

[ololololo]

We discussed ch/sh several years ago:

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It's very interesting, bit I still don't know what to think.

Thank you very much! I always have trouble finding out whether a particular topic has been discussed.
In general, the idea that sh is a contraction related to ch doesn't really work with statistics. What does it shorten? Why do the words end up looking the same? 
But we can definitely say that sh is a variant of ch, and the letter s is not involved. You see, the upward curl that's identified as part of the s can be on the left, in the middle, or even on the right (Takakari considers these cases to be cs). On my old screenshot from voynichese.com (I can't tell you what folio it's now) the curl is completely coming from below:
   
In the case of a numeric cipher, this "equality" can be explained by the fact that sh is a small deviation from ch. For example, if ch = 50, then sh = 51 or 49.