There sure were holidays to the eastern block during "the cold war". The holidays were cheaper than the usual med destinations. Yugoslavia cir 1970 flew Dan Air Comet 4 to Dubrovnik, 1971 Bulgaria Varna (think) on a Channel Airways Comet 4. Remember the Channel airways aircraft couldn`t start the engines from the Bulgarian airport, and cabin crew went down the cabin switching on each emergency light in the ceiling, and main cabin lights went off. Engines still wouldn`t start so we went back to the terminal for hours whilst they did something. The GPU cables wouldn`t fit the Comet sockets I believe?