STAMPRITE letter scale

This is a distinctive letter scale. The weight is indicated at discrete positions. Per extra ounce a separate counterweight is picked up. The photos and especially the two animations show the step-like action. Next to the indication of ounces, the chart presents the postal rates for First Class Regular, U.S. Air Mail, Foreign First Class, MDSE and Prints, Books, News en Seeds. Till now I didn't see this category Seeds. The Air Mail rate of 5c per 1oz. was valid from 1946 till 1949. At the chart is indicated patent applied for. I still don't have a copy of the patent. One may find Stamprite letter scalesproduced by different manufacturers. The scale presented here, is made by S.C. Sheldon, from Pasadena, California.