The Associated Press Stylebook (2002)entry “plurals"
SINGLE LETTERS: Use 's: Mind your p’s and q’s.
MULTIPLE LETTERS: Add s: She knows her ABCs.
New Fowler's Modern English Usage, third edition (1996)
entry “plurals of nouns,” section 10
MAs, MPs, QCs, the sixties, the 1960s. But after letters an apostrophe is obligatory: dot your i’s, mind your p’s and q’s.
John Weldon, Zola Levitt, Encounters with UFO’s
(Harvest House Publishers, 1975)