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Indeed these grainy looking hones, at least the large black one I showed and the green one are, definetely coarser than the usual water hones- thats why I said they come from a diferent quarry. But-...

Quite a possibility - sure.
But normally then the abbreviation should be BOSO or EBOSO for mentioning SO for Solingen in the name.

Well probably you got a special thuringian waterhone there, which comes from a quarry that was operated in the 1920's. The hones are black with a soemhow grainy look.
Here are some examples:
...