SCARCE BRITISH CALCULATOR - Otis King's Patent Calculator, Model L, Cylindrical Pocket Slide Rule with Helical Scales No 429 & No 430, patent no. 183723 (1921), Made in England by Carbic Ltd., London 1921-1972, stamped 'N 1162' on one end and 'Made in England' on the other of the brass case (which appears to have been nickel plated at one time). 6" long. Some wear. The model L adds a linearly divided scale section on the top, instead of two decades of logarithmically divided scale. A linear scale on a slide rule is used to find logs and anti-logs, which can be used along with multiplication and division to do arbitrary powers and roots.