Mechanical and Electromechanical Calculators[edit source]
Early models of calculator were designated by letters. The letters A, B, and C are lost in the records of those early days devoted to constructing a suitable pilot model. The "D" model started manufacture in 1915 with serial numbers below 4,000. The "E" model started manufacture in 1916 with serial numbers beginning at 4,000. The "F" model was introduced in 1917 with serial numbers above 6,000. The "G" model was the first machine of the refined style, and was introduced in 1919 with serial numbers above 20,000. The "H" and "I" were never released for production. The "K" was the real start of the big forward march by the Monroe Company. The "K" hand machine, introduced in 1921, was followed by KA, KAS, KAA, KASC, KASE, etc., machines all more fully automatic than the former. The "L" model was produced from January 1929 to February 1971. The "M" model further refined the "L".
Model 145 was the last adding machine model produced.
Model 570 was the last electro-mechanical four-function calculator model produced.