I understand there was a method of working out the allocation of the letter, it was not a random thing, it was arrived at mathematically. Our WO Adj worked it out for me one time shortly after the letter prefix came out; I had lost the paper I had it noted down on and and had a totally blank memory just at the time I needed to complete some forms.
Thanks to the
RAF Forum here is how the prefix letter was calculated (presumably the final 23 = X is a typo for 23 = Z)
Multiply the first number by 8
the second number by 17
the third number by 4
the fourth number by 16
the fifth by 2
the sixth by 13
and the last number by 1
Add the total of the answers (Answer A)
divide total by 23
reduce the number to the nearest integer (ie, 13.26087 would be 13)
multiply that number by 23 (Answer B)
take away Answer B from Answer A above
if your answer is
0 = A
1 = B
2 = C
3 = D
4 = E
5 = F
6 = G
7 = H
8 = J
9 = K
10 = L
11 = M
12 = N
13 = P
14 = Q
15 = R
16 = S
17 = T
18 = U
19 = V
20 = W
21 = X
22 = Y
23 = X