Original: Under a transitional arrangement following reunification, between 1989 and 1993 postal codes in the west were prefixed with 'W', e.g.: W-1000 Berlin 30 (postal districts in western cities were separate from the postal code) and those in the east with 'O' (for Ost), e.g.: O-1xxx Berlin.
Simplified: Between 1989 and 1993 postal codes in the west started with W, eg W-1000 Berlin 30 (postal districts in western cities were separate from the postal code) and those in the east with O (for Ost), eg: O-1234 Berlin.