An amazing handheld rotary calculator designed and built by Curt Herzstark.
When I was working with a land survey crew in the late 1960's, we used this gadget to calculate lengths, areas, and positions with absurd accuracy.
The PocketCalculator? had not yet arrived, and the SlideRule was only good to a couple of significant digits. The Curta (which resembled a small mechanical pencil sharpener) was good to 10 or 11 significant digits.