The M42 was a popular screw thread lens mount used by multiple manufacturers.
The M42 lens mount was a metric screw thread lens mount with a diameter of 42mm and thread pitch of 1mm.
It was first developed by Carl Zeiss in 1938 and used by the company's East German incarnation (Carl Zeiss Jena) from 1949 on in their Contax, Pentacon and Praktica lines. Since the mount was not patented, many other manufacturers started using it, most famously Pentax.
The M42 mount largely fell out of use in the late 1970s and 1980s.