"It soon became apparent to both Army and Navy cryptologists that a great increase in cryptosecurity would be obtained if the alphabets of the M-94 device could be made variant instead of invariant. There began efforts in both services to develop a practical instrument based upon this principle. I won't take time to show all these developments but only the final form of the one adopted by the Army, Strip Cipher Device Type, M-138-A. This form used an aluminum base into which channels with overhanging edges were cut to hold cardboard strips of alphabets which could be slid easily within the channels. It may be of interest to you to learn that after I had given up in my attempts to find a firm which would