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In pictures: The (mostly) cool history of the IBM ...
It wasn’t a mainframe but it certainly had the DNA. On April 29, 1952 IBM demoed the Defense Calculator which ultimately became the IBM 701 Electronic Data Processing Machines. According to IBM it was the first large-scale electronic computer manufactured in quantity and first commercially available scientific computer. Its programs were stored in an internal, addressable, electronic memory and was instrumental in IBM's transition from punched-card machines to electronic computers.
In pictures: The (mostly) cool history of the IBM mainframe