The ex Pan Am -500's had different avionics, autopilot & also had PMS system rather than the more integrated FMS that the ex BA aircraft had, therefore they were not that compatible. But they were going cheap and one man's -500 is the same as another -500, not when they have different avionic systems, good old MOD strike again. When it came to find money to get 706 to a flying condition, well that took awhile due to the 705 "bouncing" incident which emptied the kitty somewhat. Later the money was found and they changed the avionics so that 706 was then designated as a C 2a rather than C2.