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Jimmy I have not found any written proof so far. But what we know is - Johan Gottfried Escher founded his company in Sonneberg 1789. He sold slate products and the old (well know at this time)...

Well I cannot say if the label is really a very old one. But in general Jimmy, you are absolutely correct.
The thuringian waterwetstones are mined in slate quarries, that consist of in total 442...

Well it is definately a thuringian hone. The german designation Wasserabziehstein = waterwhetstone was typically used for thuringian hones.