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Demonstration ~ 1955 Bell Punch Sumlock Comptometer

In the early 1940's Bell Punch introduced a full-keyboard comptometer style machine under the Sumlock name which was sold by their London Computator Corporation marketing company. The name refers to the interlocking of the mechanism to prevent incomplete key strokes (the key does not return, and no other key in the same column can be depressed, until it is pressed far enough). In 1950 the name of the marketing company was changed to Sumlock Ltd.  

Demonstration Bell Punch models have been used to show how the interlocking mechanism works, so making the machines less liable to operator error.[see www.vintagecalculators.com/BellPunch/html/introduction1.html which cites also "News File", Office Magazine, May 1958, p406.