Huh. The Smithsonian also labels it a "slide rule", but I don't think that can be right. Those numbers are completely linear, not in any kind of log scale. The movable part seems to just be a 90-degree cross, which on this background is just adding multiples of 50, which seems trivial to do in one's head, even for someone like me who was raised with pocket calculators. What the heck was this used to *do*?