As pointed out by Hermione and Halcyon on sulekha.com, Ross Beresford's list of letter indicators used in cryptics (from rec.puzzles) includes some mysterious ones - e.g., the letter a is indicated by use of "five hundred" or "five thousand" in a clue.
I've seen the use of 'd' (the roman numeral for five hundred) as in
Landlord expresses puzzled wonder without five hundred. (5)
'Q' also could be indicated by five hundred, etc., from the Latin stem 'quin-'. 'P' could arise from the Greek 'penta-'. A monkey is slang for a 500 British Pound note, so perhaps 'm' might be indicated by 500. Search of Gaelic and Cockney dictionaries online does not reveal any a-word that would fit. Perhaps Chambers Crossword Dictionary explains this?
Anybody with a clue, please write!
(note: the answer to the above clue is owner, anagram of wonder without the d).
Postscript: Halcyon sent this:
" In the Middle Ages they sometimes used the rest of the alphabet
for extra numerals. This appears to have been a pretty slapdash
requisition, as there are repeats and gaps, and the order doesn't
make much sense. I have no idea how widespread this usage was, if
at all, or whether it was completely esoteric.
S = 7 or 70, O = 11, F = 40, A = 50 or 500, S = 70 or 7, R = 80,
N = 90, Y = 150, T = 160, H = 200, E = 250 and K = 250, B = 300,
G = 400 and P = 400, A = 500 or 50 and Q = 500, Z = 2000.