Great to hear that these are usable stones! The stone I was thinking of when I said the grit might be 6k was from Hida Tool, my mistake.

That website doesn't specify the grit rating on the nagura, but I wouldn't be surprised if it were aiding in the polishing effect that you describe when using a slurry, since the same stone would be used for even finer polishing stones.

It is commonly said that Japanese stones become finer and finer with use because the grit particles break down as they are exposed. While many other stones have grit particles that are hard enough to keep their size and shape as they are used.