The only thing I can tell you, (not sure it'll be helpful) is that Norton still uses a simliar coding system for their Arkansas stones. An HB13 is a 4 inch translucent, an HB14 is a 6x2 translucent I think. So it is possible Indian Mountain whetstone was operating under Norton in some fashion. The other simliarity that kinda jumps out is just the Indian Mountain part. As opposed to Ouchita, or Washita Mountains (Original and westernized forms), which is where the Arkansas stones are mined. But anyways, I'm just brainstorming here. I don't have any facts for ya. Hope I helped a little.