One-of-a-kind computers

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The category of one-of-a-kind computers contains computer systems of which only one unit was ever produced. Such systems have typically been experimental prototypes and/or too massive, complex, and expensive to duplicate. Among these systems are the pioneering electro-mechanical and electronic computers made in the 1930s, 1940s, and 1950s, and, after that period, the largest supercomputing systems.