The car was initially designated in 1973 as the Iso–Marlboro IR and two examples started the season. One was destroyed in an accident at the 1973 German Grand Prix, and a replacement was built. When Marlboro and Italian sports car manufacturer Iso both withdrew their backing before the 1974 season, the two cars were renamed for 1974 as the Iso–Marlboro FW. Another example was completed during the 1974 season. These three surviving cars were renamed as Williams cars for 1975, and carried separate designations: FW01, FW02 and FW03, although the cars were of the same type.