Peter, that's a fact!
I will find out in time what it does for me. At the moment I have not shaven yet on a finish produced by this actual stone, but I did finish some french pocket knives with it. It behaved like the other dark Thuringian I have, equally fast and maybe a slightly better result, but this may be because it is slightly bigger and therefore easier to use.
I am pretty sure this hone is not another slate type, at least not the ones I came across.
If there is no definite answer to whether it is an Escher or not, it will be so. At least we contributed a little to the knowledge base of Thuringian hones in general.
Thanks guys, I hope we will some day be able to clarify the mystery.

Greetings, Han