The heavy series Unimogs were introduced in 1974, and first featured the edgy cab, which is still a design feature of the Unimog today. The first heavy series Unimog was the 425-series, which was available from 1976. Soon after, the 435-series and 424-series followed, which caused a decline in Unimog 406 sales.The 425 was available with a wheelbase of 2,810 mm, the 424 with 2,650 mm and 3,250 mm, and the 435 with 3,250 mm, 3,700 mm and 3,850 mm. The introduction of new engines starting in 1986 caused a shift in the series numbers, but leaving the vehicles mostly unchanged otherwise. The 424 became 427 and both 425 and 435 were joined together and became 437. A derivative of the 437-series, the 437.4-series is still in production today.