2 digit was used from 1944 to 1961
4 digit was used from 1961 to 1993
5 digit from 1993 on
everything before 1944 was adressed only by the Name of the Place and the Area, like Blablatown in Franken or so.
Just to add to uscob's useful Postleitzahl [=zip] information, of course Berlin had a 2 digit postal coding system in operation before 1944, in conjunction with points of the compass.
As one example, the Kreuzberg area was coded SO [=Süd-ost] 36.
Wouldn't want anybody to discount e.g. what might be a perfectly good helmet because the liner band is marked with a date before 1944 & Metall-Verarbeitungs-G.m.b.H. Berlin-Charlottenburg 2alpha+2numeric zip...
The same applies equally well to other items made in Berlin & marked with the maker name & location.
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