Has anyone used the Shapton HC (High Carbon) stones? Can anyone clarify the difference between HC and not HC?

You can find these on both the Glass and the Pro stones. The best that I could find is that you should get the HC if you find the "not HC" version to be unsatisfactory with your high carbon knives.

My best guess is that there are (I believe with almost no conviction) that there are High Carbon Japanese knives that are very hard (and I think have very small particles). I guessed (also with no certainty) that the HC stones might be slightly reformulated to work with harder steel.

So I am curious if a person who does not have a stone and would it make any difference if they purchased the HC version even if they did not need it.

I did find at least one mention on the site here, but that was years ago with nothing concrete stated.

I do own one knife with particularly hard steel and I own PM-V11.