The stone is old(ish), his father bought it some 50 years ago in Germany. I'll snap a pic of the box when I get home again.

This morning I tried it with light slurry. I had lapped it with Atoma 400, found it to be very close to perfectly flat, so it didn't take long to finish. Then finished the surface with 1200 (also diamond). While at it, I put a razor on it and did around 50 laps on light slurry. This is the result, with almost no pressure. The edge does pop arm hair, but is no good for shaving yet.

I will take your advice and now go to clear water, and do some sets od 50. Thank you!