Mercur (or Merkur) is what I was looking for.
In his autobiography “Meine Welt” Rudolf Caracciola describes how he, as a sales representative for Fafnir, wanted to enter a Fafnir for a light car race in Berlin. Because of his bad sales results he was denied a car. Instead he borrowed a friend’s 4 hp EGO, but before he could enter the race the little car had to be overhauled by the factory. After a hectic race Caracciola won first prize (of course!). The name Caracciola still is well-known, but almost nobody remembers the EGO. The car was made from 1921-1927 by Merkur, an airplane factory east of Berlin.
