There were indeed six. AOYH, YL, YO, YR, APEX and EY. I'd seen then all at LBA within a fortnight or so of starting collecting reggies in August 1974.
Don't know about earlier operations but by that time I'd think all six were used on all routes.
Some maintenance was done at Leeds but ISTR major overhauls were at Cardiff by then.
EDIT: Routes then were LHR, BFS, JER, GCI, DUB (pooled with EI) and AMS. Paris and Brussels had been tried but dropped following the Yom Kippur war and consequent 'fuel crisis'. The Channel Islands were summer only.
AMS was only three rotations a week and NE dropped it in early 75. Air Anglia stepped in immediately upping frequency to double daily at business sensible timings at first opportunity. There is then a continuous line over next 40 yrs via Air UK, KLM UK to the current City Hopper service.
Last edited by Airbanda; 12th Jan 2015 at 16:58.