This Model C Conto calculator, serial 2175, was manufactured in ~1922 by Carl Landolt in Thalwil, Zürich.
The machine is easier to operate than a stylus based lightening type calculator, since knobs are used for input (oddly being rotated anti-clockwise), but it is not possible to do direct subtractions with it (and there is no provision for complementary arithmetic). A window is provided to check the numbers as they are input. The Crescent-shaped slider at the top is moved to the right to perform an addition, the other lever being provided to bring the register back to zero. The machine is heavy and solid and this one still works reliably.
The machines were produced by J Aumund, in Zürich from 1912, having been patented by him and Karl Karasek in 1905 and 1906 (patent CH 33474 + 38362). This was followed by patent CH 60809 in 1913. Carl Landolt took over production of the Conto in 1912.1
Carl Landolt was born in 1869 in Colmar, dying in1923 in Thalwil. From 1888 he worked as a statistician in Zürich and in in 1910 founded his company for the manufacture and trade in calculators.2
The machines were produced from 1912 to the mid-1920s by Landolt and after that by Alfred Müller.
1 http://www.rechnen-ohne-strom.de/rechner-galerie/addiermaschinen/scheibenaddierer-metall/ viewed 8 Mar 2014 (↑)
2 See Markus Bürgi “Landolt, Carl” in http://www.hls-dhs-dss.ch/textes/d/D48492.php viewed 8 Mar 2014 (↑)
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