I have a Dietzgen rule with "folded" C and D scales which enable reading to 4 decimal places and specialized scales for solving triangles. I also have several circular rules, including some specialized ones for aviation complete with WWII QA stamps, and my favorite, a "Nuclear Bomb Effects Computer, Revised Edition"; In the movie, this was used by Dr. Strangelove to calculate the half-life of "Cobalt-Thorium G" after it was apparent that the "Doomsday Device" was going to be triggered.
73, Don N2VGU
Donald S Brant Jr