@Ploughman
I'm glad you asked me that
We (the then active members of the research group in the mid 1990s) spent many hours in the pub, and in the Bewley's Cafe Museum (long gone now) trying to work out the pattern for the ETO war codes.
There isn't one that we could find. Doesn't mean that there isn't one, just that we couldn't find one.
(They don't spell the name of Brennan's grandmother or something backwards either, though we didn't actually check that one out. We did check actual backward progressions, skipping letters, skipping groups of letters, arranging the letters in a circle, or a triangle, or a square-we really did do this! All of this was done, mostly on serviettes, or beermats and the like, to encourage the flow of ideas, etc. Tada, nada, nic, nothing, rien. It was good craic though, not your usual pub night.
Available data suggests that the code letters were chosen at random. Not the answer we want.
I, O, Q, U, X, and Z were not used. The first three might have been excluded because of resemblence to numerals, as they were excluded from use as prefix cyphers on then banknotes.
EDIT: U, X, Z may have been allocated, but not used due to the Emergency coming to an end first.
