Though some tout the Arithmo as the very first timepiece to incorporate a slide rule mechanism it in truth followed the release of Mimo's Loga
link and the Breitling Chronomat. With that said I'd argue that the Arithmo has a leg up on the rest thanks to its truly wild design. Most notably this reference
link makes use of a second convex crystal which extends
link over the bezel. This detail is lost when looking at the watch in head on photos though at an angle its magic becomes evident.