POSTAMATIC letter scale

This spring balance very strictly is a letter scale. The scale presents next to one another the rates for letters and for Air Mail. A division in ounces is not given. The range corresponds with 4 ounces. For letters this postal scale starts at 4 cents per ounce. The Air Mail rate starts at 7 cents. This letter scale was made in more colors: red, yellow, green and blue. According to Bill & Jan Berning, [see literature], also clear specimens were made. I still haven't seen one. Different postal rates on the Postamatic are found: 3 cents (1932-1958), 4 cents (1958-1963), 5 cents (1963-1968). A specimen from 1968, with a starting rate of 6c, doesn't present this rate on the scale itself. The postal rates are glued into the lid of the box of the scale. Berning states also that specimens exist with a 2 cents first class rate(1919-1932), I still have not seen an example of this. On older specimens the abbreviation LTRS (Letters) is used. Specimens from 1958 and more recent, have the abbreviation 1ST (First Class).