All two letter codes back then
IIRC BKS (BK) was Barn
aby, Keegan & Stevens
OU = Autair also used the last 2 letters of the reg.
BY = Britannia
BE = Beeline BEA just used the registration, as did BA = BOAC
RS = Ascot
RR = Rafair (remember details passed as "a military twin Jet" "Roger, Dominie")
BM = BMA (Midland)
PA = Pan Am
and many more, now forgotten. I think we had to learn the whole book back in the 1960s when I started at LATCC