No, the BBW's slurry would be violet / red, and this one's pure grey.

I just used the diamond stone on a BBW before lapping this one, so there is maybe a little touch of red on the picture's slurry, but it's definitely not a BBW.

I don't think of it as a french slate (the color would be more like a Dragon Tongue's). Maybe a thin Thuringe ?

The thin stones I was thinking about were Sweden's by the way, but I can't find back the picture I saw that reminded me this one.