At the end of the Second World War, the IRS was handling sixty million tax returns each year, using a combination of mechanical desk calculators, accounting machines and pencil and paper forms. In 1948 punch card equipment was used. The first trial of a computer system for income tax processing was in 1955, when an IBM 650 was installed in a Kansas City office that processed 1.1 million returns. The IRS was authorized to proceed with computerization in 1959, and purchased IBM 1401 and IBM 7070 systems for local and regional data processing centers. The Social Security Number was used for taxpayer identification starting in 1965. By 1967, all returns were processed by computer and punched card data entry was phased out.
-Source: Wikipedia Internal Revenue Service page