They were by area, however there was little meaning to most of them. It's something like:Something that has always puzzled me about Crosville is there area prefix letters. Were they suppoed to coincide with geographical areas or not? I was once told that D was for Denbighshire, but that doesn’t quite fit with their routes as I remember them, certainly none of Shrewsbury (D55, D70. D74, D75, D77), Ellesmere (D55 and others), Oswestry (D53, D63, D70, D71 and others) have ever been in Denbighdhire though I can see how C and K could relate to Cheshire and F to Flintshire. However I’m then not sure how A, B, J or M would fit.
A Flint
B Mold
C Chester
D Wrexham/Oswestry
E/G Wrexham
F Wirral
H Liverpool/Warringhton
J Runcorn
K Crewe
L Limited Stop
M Rhyl/Llandudno
N Bangor/Caernarvon
R Caernarvon/Porthmadog/Bleanau Ffestiniog
S Barmouth/Dolgellau/Aberystwyth
T Runcorn Busway
V Ex Vaggs Motors
X Express