Complex sentence: In 1969, Kawasaki introduced the Mach III H1500 which was the fastest production bike and retained that title until 1972 when Kawasaki introduced its big brother, the Mach IV H2750.
Split into simpler sentences: In 1969, Kawasaki introduced the Mach III H1500. It was the fastest production bike through 1969--1972 until Kawasaki introduced its big brother, the Mach IV H2750.