In 1976 or 1977 (I forget the exact year) the code in the UK changed from 3100 to 7500. I remember the day it changed well. The first flight I had from Humberside to Amsterdam was allocated code 3100 by LATCC. I passed it to the Air Angila flight who took it without question. Some hours later the pilot phoned from Amsterdam wanting to know why I had given him the Hijack code. Apparently Amsterdam made him number 1 with a straight in approach (this was during the morning rush). After landing he was directed to a remote stand and the aircraft was surrounded by the military. Although the code had changed at midnight no one had reset the Amsterdam computer to recognise the new code so HJACK appeared in the radar label.