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The stone looks a lot like a Schwedenstein. Marked, they are almost always a "combo" with a thin, no more than 1cm piece of stone on one side, and leather on the back, on a wooden box/paddle like...

I bought a 20cm piece a week ago from a dear friend. He too thinks that they are as good or bit finer than a classic Thuringian, but faster and harder.
And, I've also seen a piece that was...