The "Hydro-Electric" system (windows, front seat adjustment and convertible top) was standard on 1947 model year. The seat and window assist system became available on GM closed cars (standard on some Cadillac Series 75 models and all Series 60 Specials, commonly called "Fleetwood" beginning with the 1948). The full system was standard only on the high-end GM convertibles made by Oldsmobile, Buick, and Cadillac. It was only available as a package; that is, power assisted windows, front seat, and convertible top (where applicable). This feature can be identified in 1948 and later General Motors model numbers with an "X" at the end, such as the 1951 Cadillac Sixty Special sedan, model 6019X. The electrically operated hydraulic pump system was shared by Hudson and Packard for their 1948 through 1950 models. The driver's door contained four buttons in addition to the remaining individual windows.