letter opener & letter scale

I don't know the origin of this combination of letter opener & letter scale & ruler. The chart is easy to read, some values are somewhat odd: 2,5 grams is hardly ever specified at the chart of a letter scale. A letter is clipped in the leaf spring at the end. By shifting the runner one determines the equilibrium and finally reads the value under the red line of the runner. A nominal little part of the transparent plastic is broken away, this does not hinder sufficiently reliable letter weighing with this specimen. At the back is a ruler of 100 millimeters. Below some more pictures of this combined letter opener and letter scale.