Germany began using postal zones to route mail during WWII. The postcodes were initially 2-digit numbers, with single-letter suffixes used when a zone was subdivided. This system was maintained after the war, with the BRD and DDR both shifting to more modern, 4-digit ZIP-Code-like systems in the late 1950s to mid-1960s. The 4-digit system has since been replaced by a 5-digit system.
This map is from Michel, and shows the zones at some time post-war, but before the switch to a newer system.
