Production year of the CZJ Tessar 180 lens

[leicacopy]

Hello,
could anybody tell me please, how can I find the year of production of my CZJ Tessar lens?
On the front ring of the lens is: Carl Zeiss Jena Nr.2814331 Tessar 1:4,5 f=18cm
On the side - N51

Thanks
Alexander

[Dan Fromm]

1942 or so.

[Sevo]

Anywhere between 1942 and 1948 - they hit the 2.7 million in 1942, but they numbered lenses by batch, and by the later war years, many batches were postponed, with some numbers reused in the post-war period.