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If it's something like this, chances are it's a coticule. These are 7 different stones. AC, variuos belgischer brocken, some newer, some older. The variety of the backing (mostly slate/black shist...

Looks like a coticule. Could even be a good one. The back seems to be slate, so pay no attention to it at this moment.

I would dress the "yellow" side with abrasive powder (if it is not straight,...