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Looks like mark was spot on on the hone. These can actually be obtained these days (still produced in Ardennes), although not too often and the price point would be about $80.

You should be able to use a regular slurry stone (which I believe is a small piece of belgian yellow) on both sides. I personally like to use the same rubbing stone as the main stone. It looks like...

these are not in production so there are no prices on them. consider the prices what you paid for them, or if you intend to sell them (which it doesn't appear you do) you can find the market price on...