Constant improvements to the original model led to the 401 series, which was produced from 1953-1956. This model series also introduced the numerical designation 400, which is used for all Unimog model series to this day. Model variant designations were introduced at the same time, allowing exact identification of vehicles. The first six digits (model series/model variant) were also the start of the chassis number. 10,928 units in 5 model variants were produced. The 402 series was introduced at the same time as the 401 series, and had a 400 mm longer wheelbase. The 401 series was the first series to bear the Mercedes star as a trademark, replacing the oxhead symbol in September 1955 after a period of parallel use.