Obviously I'm 'a bit slow on the uptake', although I have to confess to never actually being confused about which was the newer ac, possibly because I knew that the K was really an E with H engines anyway. Although I was a little confused as to why we called it a K in the first place, particularly as we then designated it C1/ C3 (short/ long) and then the J (manufacturer's designation) as C4/ C5 (long/ short).