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http://straightrazorpalace.com/hones/43768-salmens-natural-oilstone-yellow-lake-brand.html

Thats one thread, which does mention the Salmen hones, its more the size and the fact you got the stone...

I would guess its a Welsh slate, I have plenty 8 x 2 x 1 inches in size, all good stones smiths honing solution works great also, for me anice easy stone to use, some i have, have been in the card...