That could be a reglue to a slate of some kind that is usable. The glue used looks similar to what Ardennes uses, I would hazard a guess it is one of their older ones.
You can get just a slurry stone from Ardennes or The superior shave.
My preference for lapping coti's is to start low at 150 grit sandpaper if there is any serious dishing, gouges or chips that would effect honing. Then move up to 800 or 1000 grit. Some of the super hard ones will take a little longer. Then rinse the stone off under running water, rubbing with your palm to remove any stuck on grit or slurry.
I try slurry here and there, but am never really that successful with it on any of my coti's. I get my best results taking a razor to an 8k and honing until the edge feels and sounds smooth over the edge. Typically 100-200 laps with a lot of water.