A Comptometer is a mechanical calculator that was produced from 1888 to 1961 by the Felt & Tarrant Mfg. Co..
This is the 1958 edition of the basic manual for using the Comptometer. It is virtually identical to the 1940s edition, except that all photographs and drawings were updated to new models, and the page about the Controlled-Key (now the Accuracy Key) has been rewritten.
In 1957 the Felt & Tarrant Mfg. Company changed its name to the Comptometer Corporation.