Single letters aren't generally valid on Apterous. A is, but it's the only one. However, they all have their own entry in the dictionary, in lower case. To give an example:
It's just listed as a noun with a plural. And they're all like that. So as far as I'm concerned, all letters and all letters + s are valid on Countdown. So as, bs, cs, ds etc.Q2
(also q)
NOUN (qs, q's)
1 The seventeenth letter of the alphabet.
Then there's Roman numerals. If you look up the entry for e.g. "one", it says:
i is valid anyway (see above), but being an alternative to one makes it doubly so. But anyway, the following are all explicitly listed under the entry for the appropriate number as alternatives in lower case, so should be valid: ii, iii, (weirdly not iv as far as I can see), v, vi, vii, viii, ix, x, xi, xii, xiii, xiv, xv, xvi, xvii, xviii, xix, xx, xxx, xl, l, lx, lxx, lxxx, xc, c, m. So basically integers up to 20 (except 4), then 10s up to 100, and also 1000.(also i, I)
This is all from the free version of course, and the paid version might be different of course (though I'm guessing not).
Discuss.