This car was presented in 1955 at Frankfurt's IAA. The first 11 preproduction units came to Bundeswehr in January 1956 while production started not until October of the same year. It was named "M-Wagen" (Mehrzweckwagen = multipurpose car) as it was planned for civil use also. As far as I understand the name "M-Wagen" included the steel bodied production cars as well.
(source: Kittler, Rönnberg; "Deutsche Autos, Offroader und SUV")