I think I've figured out their NAG call sign/flight-number system : the letters don't change but the numbers do : they are incremented with every movement : to Kabul they're at 67 whereas from Kabul the number is 26, which is weird because you'd think that if you fly in you also have to fly out so the number would be the same either way. Unless of course they're totals : total RAF NAG flights to Kabul and total NAG flight to Akrotiri.