I'd send a PM to A_S to make sure he sees it. He is very familiar with obscure natural hones from around the world. IIRC there has been mention of coticules harvested adjacent to Belgium. It is possible Alex might be able to give you something to go with by studying the photos.

One way or the other, all we can do is take razors to them with and without slurry. At various stages of keenness and see what we come up with. Beautiful rock though.