In October 1961 a Russian-speaking liaison officer on the staff of the Provost Marshal, moved through Checkpoint Charlie in a blue Taunus, escorted by three Jeeps filled with troops wearing bulletproof vests, while tanks idled their engines at the white line that marked the border. After a five-minute trip, the officer returned, flashed his headlights in a signal, and was again escorted by Jeeps to West Berlin.

Source: Wikipedia