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That's my verdict but you should get other people's opinions as well.

To me it looks like a Coticule natural bonded with BBW at both sides and backed with slate. You can't just lap away the BBW layer at the surface because you will reach the bottom layer in no time....

BBW stands for Belgian Blue Whetstone. They are usually slower and coarser than Coticules. Coticule veins go through BBW so some Coticule whetstones are naturally bonded with BBW, others are backed...

Can you show the sides of the stones? It looks like the BBW side is coming through, if that's the case I would return it.