Otis Carter Formby King (1876–1944) was a grocer and engineer in London who invented and produced a cylindrical slide rule with helical scales, primarily for business uses initially. The product was named Otis King’s Patent Calculator, and was manufactured and sold in London from about 1920 to the 1970s. This example bears the patent number 183723 which dates it to 1921. It measures 25.5cms when fully extended and fits into its original case measuring 15.5cms. The case has a little expected wear to the top fasten which can be seen in the pics.