I cannot find a reference (image) on this (exact) refractometer on the web. Obviously very old and far as I can tell will work great, all the parts and lighting/index wheel work like a fine tool you’d expect. I am not a patent search expert so the #178 patent pending means little to me. Some of the GIA literature points to them (Shipley I think) inventing some of these back in the 1930’s ish and beyond but this model is elusive, maybe the precursor to the more popular Duplex II. Anybody have more information on these units if they were widely manufactured/used?