The FBK and RSD worked together for security and protection during trips and public events, but they operated as two groups and used separate vehicles. RSD chief Rattenhuber was in overall command and the FBK chief acted as his deputy. His FBK chauffeur Erich Kempka usually drove one of Hitler's black Mercedes-Benz cars from a fleet of six to eight that were stationed in Berlin, Munich and other places. Unless in the company of an important person, Hitler would sit in the front seat next to Kempka, with an adjutant behind him. When travelling in motorcades, following Hitler's Mercedes would be two cars to the left and right, one with FBK men and the other with a detachment of RSD men. In July 1938, upon Kempka's directive a fully armor-plated Mercedes was built and delivered in time for Hitler's 50th birthday of 20 April 1939. The car had 18 mm steel plate and 40 mm bullet-proof glass.