There is some indication that for a period this term described stones from Shoubudani or Nakyama. But these days widely applied to a wide range of Kyoto area stones. Best not to read too much into that stamp.Does Shohonyama translate as true main mountain? Does anyone know the mines this is referring to? I also see shomotoyama but I think that is the same thing and is just a google translate issue.
Not to say there aren’t fabulous stones bearing that stamp, there certainly are, but it doesn’t reliably tell you a whole lot about the stone.