A Keuffer Ruler is a manual sliding ruler that served as a vintage calculator, probably dating back to the 1940s. It was manufactured in the United States by Keuffer and Esser CO N.Y., and was government-regulated by the US Beauru of Standards. By sliding the middle section to align with the top and bottom, various functions could be performed, such as long devision, square rooting, multiplication, and so much more.
Keuffers Ruler were so complex, they required an instruction manual to be utilized.