My understanding is that that yes in the UK we continued with the K2 model only and did not receive the K3,K4 and K5 so the frame No's for the UK always remained in the 2000000's series.
However, the engines used were simply from the current manufacturing stock so whilst the frames remained in the K2 series the engines used for each new production year would follow the new K for that year so that in 73 they used the K3 series engines starting with 2200001, then for 74 the K4 series engines starting 2300001 and so on?
If that is correct then the indication is that the bike that I am looking at appears to have had a 74 K4 engine fitted at some time?