Designed by Swedish cryptographer Boris Hagelin, the M-209 Converter was a portable, mechanical cipher machine. Used extensively during WWII and the Korean conflict by both the Army Signal Corps and the U.S. Navy (which designated it the CSP-1500), Hagelin’s device allowed a rapid and secure means of communication. The lunchbox-sized machine was so simple that little training was required to use it, yet its cipher scheme proved very difficult - but not impossible - for the Germans to crack. The M-209’s elegant design uses six adjustable key wheels or rotors to produce a reciprocal substitution - also known as a Beaufort cipher. The effect was that of sliding a normal alphabet sequence against the same sequence reversed, with a high degree of irregularity in the shifting process brought about by a train of gears. In the train of gears, the number of teeth on the driving number was also varied by the use of keying elements. Prepared by the War Department for the Signal Corps and other users, this M-209 technical manual describes the operation and care of this fascinating device. Originally printed in 1942 and classified restricted, it was declassified long ago and is here available in book form.