I see what you’re saying and that is why I was mentioning not consecutive order of the serials, but assumed that say X1154 would be A, and if X1156 was the next serial on strength it would be B and not C, and that codes were applied on unit when all allocated aircraft were on strength.
However it does seem a hard system to apply, and an even harder one to maintain (especially in war periods) so you could well be right.

Edit, As I say I’m making assumptions here and can’t really research at the moment, but this thread on Lightnings, http://forum.keypublishing.co.uk/showthread.php?t=103936&highlight=56+lightningson, is a good example that illustrates what I’m trying to get at, particularly post #7 and #15. VC-10s and Dominies are other examples I can cite straight away, I do realise these are all well and truely post-war though. Interesting topic!