RLM aircraft designation system
The German Air Ministry (Reichsluftfahrtministerium; RLM) had a system for aircraft designation which was an attempt by the aviation authorities of the Third Reich to standardize and produce an identifier for each airframe type produced in Germany. It was in use from 1933 to 1945 though many pre-1933 aircraft were included and the system had changes over those years.
Source: Wikipedia — RLM aircraft designation system (CC BY-SA 4.0)