Among Arne Cröll's excellent publications here: https://www.arnecroell.com/publications are two on the post-World War II Zeiss companies, the one in the original location in Jena and the other in Oberkochen in West Germany. As Havoc says, it's complicated, especially how the two lens makers' names were rendered. I think Arne does a good job of sorting this out.
Based on Hartmut Thiele's Fabrikationsbuch Photooptik I Carl Zeiss Jena, Zeiss changed from millimeters to centimeters in describing focal length about 1910. Carl Zeiss Jena switched back to millimeters about 1949-1950. Also from Cröll (and other of Thiele's works), after WWII Carl Zeiss Jena continued using the serial number series that they started in the late 19th century. By 1979 serial numbers were in the ten millions. They began anew with serial numbers in 1980. Zeiss Oberkochen had their own serial number series.
David
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