I seem to remember that it was not until after the C2s were purchased was it realised that not all -500s were the same.......
It's correct that there are some differences between the KC1's,K1's (which where built for BA) and the C2's, due to different Airlines wanting different specks (BA bought the GL version where the Yanks bought the Ghia). There were even more differences after Marshalls had finished butchering the KC1's and K1's. The C2's originally had digital autopilots which where removed and replaced with an analogue unit to make them standard with the others!! You could understand this if they had done this with everything, unfortunately there is a reason you have to do an "Aircraft Differences" package before you start the Flying phase of the conversion course.
The 3 new aircraft where being looked at just before I was posted, the rumour was that due to the FSTA contract, the RAF was prohibited from acquiring any other Large passenger jets, so the aircraft were to be purchased by Marshalls and leased to the RAF for a fee. No sure how true the rumour was. The one thing that was certain was that if the new jets did arrive there would be no increase in the number of crews.