From 1966 the Unimog porfolio was significantly expanded in different output classes. The 421 series was developed to close the gap between the 411 and 406 series. It remained in production until 1989, with a total of 18,995 units in 20 model variants which included traction head versions. Once again there was a choice of an open or enclosed cab. With the exception of the floor assembly, the enclosed version was the same as in the 406 series. The series was powered by modified four-cylinder diesel engines from the passenger car range. The 431 series was produced in Argentina on the basis of the 421 series. Assemblies from the 421 series also provided the basis for special-purpose vehicles by other manufacturers such as Werner Uniknick or Intertractor UT45.