Just like the other post this week I found a Coticule combination stone (4"x1 3/4" yellow/blue) in my late parents junk drawer. I've lapped it with my Norton flattening stone and it does have the purple slurry so I'm pretty sure it's a a y/b stone.

What would be the best way to lap this? I'm not sure the Norton is the best choice, also I don't have the matching stones to make the slurry, what else could I use?