The Carborundum Grinding Wheel Speed & Safety Indicator

Here's another non-wheel, but interesting nonetheless. This slide chart was copyright 1939. The sliding bit is of cardboard while the outside is a printed plastic. This company is still in business and makes things like sanding belts and disks -- abrasive things! This chart measures 3 by 6 inches.

I like the design of this chart -- the typeface on the title, the Art Deco edge and, of course, the illustration on the back. I can't make out what the little round logo is and I even looked under my microscope. Maybe it's supposed to be a little globe. When you slide the slider, the little windows on the front show up red or green. It doesn't specify what the colors are, specifically, but I'm guessing it's level of danger. ? The cardboard slider has information on both sides, as you can see. If I knew anything about grinding wheels, I would address that here.