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Introduced late in the '71 model year, this optional breakerless ignition system was the forerunner to future full-electronic distributors. Not without its share of shortfalls, the Unitized distributor was a popular option until being replaced by the High Energy Ignition system in mid-1974. With limited availability, optional applications were limited to the '71-74 L75 455ci, '73-74 L78 400ci, and the '72 455 H.O. Unitized was not available on the Super Duty 455 for '73-74.