front of Multi-Position Letter Sorting Machine
Multi-Position Letter Sorting Machine

Image courtesy of Smithsonian's National Postal Museum
The exhibit utilizes a computerized representation of a MPLSM (Multi-Position Letter Sorting Machine) in which visitors are challenged to hit the right keys (first 3 digits of a letter’s ZIP code) to process their pieces of mail. The MPLSM machines were used with the introduction of ZIP codes in 1963 to move more mail, more quickly. A dozen workers stationed at each large machine would “key in” number codes for each letter as they passed by, more than 60 per minute. The letters were then sorted by their codes into bins accessed from the back of the machine.