I've seen claims that the "house brand" resinoid stones distributed by "Metal Master" are sourced from the same manufacturer as Ohishi and may be the same. I have the MM 10K and I've used the Ohishi 10K, and they're very similar in terms of scratch pattern and texture/feel. Beyond that I have no direct information.
With all of these different makers marching to their own specs, how does one know if this 10K is any finer a medium than another maker's 8K?
To the best of my knowledge my 8K Norton stone has 3µ abrasive particles. I am sure smaller particles would make a slightly sharper edge, but is it really worth it? My skill level seems to do fine with 3µ. It doesn't seem getting sharper would really improve much other than my ego.
jtk
"A pessimist sees the difficulty in every opportunity; an optimist sees the opportunity in every difficulty."
- Sir Winston Churchill (1874-1965)