DrSyn,
Bow to your knowledge on the origin of the 51 Comets. All 4 of them had numerous chinagraph or paint BOAC markings, which were, I assume from what you have said, appended during manufacture. I assumed that they were ex-BOAC. As far as I can remember, the four a/c mentioned, all appeared on 51 around 1960/61, although it might have been earlier. XK 671 was the only one of the four which was kept in its original commercial configuration and used to ferry squadron personnel around the globe.
Happy days!