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If I were to guess, I would say a Arkansas stone of unknown grit & would bet that it's not a finisher, which is what I am sure you were looking/hoping for.

How much are they wanting for it? I know price is relative, but I too would have to say it's more than likely a Arkansas stone used for knives as Kelly mentioned. There are better stones for...

With all that oil on it, it's hard to tell. You will need to lap it some so that we can see the actual stone, then post some new pics.