The stuff he seems to be getting is, interestingly enough, not available at any of the retailers I speak to or woodworkers/knife guys I know in Japan. Either that or they're being tightlipped about it. Most of the people I talk to show me their incredible collections and have been very helpful when finding stones or knives that I'm looking for.
With that said, I've asked friends specifically about his Yamashiro stones with the bright blue stamps and none of them have ever seen them. Makes me think that he has a dealer that's specifically stamping for him to let them perpetuate the story of them being stamped by a "temple monk" or however it goes.
As far as the Nakayama Maruichi. I've owned many of these stones in the past but there is very little information about the authenticity of the stamps. They've all been top notch razor stones but none were what I'd look for in knife or tool stones. There's a story about a previous owner and a Hatanaka connection that's convoluted and impossible to trace in English. The only other two places that sell the maruichi that I've seen are japan-tool and JNS.
Either way, his access to iromono stones has me drooling.