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It's not a Special Stone, this is an example of a Pierre La Lune Special Stone and here is one of many threads about them:

http://straightrazorpalace.com/hones/96159-pierre-la-lune.html

191723

I don't know if that's the La Lune or Special Stone, "pierre" en francais means, "stone", and with the name Solingen, this could be an average razor stone/hone...let's see what more of the HAD...