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In 1957 Jacob Rabinow’s company, Rabinow Engineering, designed a mechanical coding and sorting system for the U.S. Post Office. Based on similar work Rabinow had done previously for the Census Bureau, the system for the Post Office consisted of a conveyor belt cart designed to carry 12 envelopes in separate compartments. Each compartment in the cart was accompanied by an escort memory. The escort memory consisted of a shaft carrying a series of nylon code wheels. Each of the 134 wheels could move laterally into one of two positions. With this combination, more than 8000 (in decimal notation) binary numbers could be set. Twelve keyboard operators would read envelope addresses and set the positions of the code wheels. The wheels were supported on a track also composed of binary elements. When the encoded compartment hit the same encoded position on the track, the shaft dropped, triggering a door release in the compartment holding the envelope. The envelope then fell into the appropriate pocket. About 600 of the letter sorting machines were built and used by the Post Office. In later operations optical character recognition technology replaced the keyboard operators, and the mechanical escort memory was replaced by computer memory.