1959 - 1967
When NSU resumed car production the NSU-Fiat marque had to find a new brand name. They decided on the name of the Neckar river near their works at Heilbronn. It was used for assembled Fiat cars up to 1967, when it was dropped in favour of Fiat for the cars assembled at Heilbronn.
Source: The Beaulieu Encyclopaedia of the Automobile