"In 1948, at the request of the computer group at Northrop Aviation, Inc., IBM coupled a 603 calculator with a 405 accounting machine, using the 405 as a control unit and printer.  This combination was so successful that after the 604 came out, Northrop approached IBM with a proposal and request for a more powerful version based on the 604. IBM responded by developing the card programmed calculator (CPC).

The CPC was made up of three units: a 605, a 417, and a 941. The first two were modifications of the 604 calculator and the 402 accounting machine, respectively, and the 941 was a special box containing 480 digits of additional storage using the same type of electromechanical storage used in the 417. Programs of almost any length and complexity, punched into IBM cards, could be fed through the reader unit of the 417, with the mathematics performed by the 605, the intermediate results stored in the 605 or 941, and final results punched into cards in the 605's punch unit, or printed by the 417 print unit. Almost 700 of these combinations were built. As Rosen wrote (1969),
     The CPC was slow by electronic computer standards, running at a maximum speed of 150 instruction cards per minute. Yet it is hard to exaggerate its role as an interim computer, carrying the major computing load in dozens of computation centers while they were waiting for the stored program computers to live up to their promises in terms of delivery and performance." [Annals of the History of Computing,  Vol.2  #3  July 1980]