New York Periods: Use apostrophes for plurals of abbreviations that have money letters and durations: M.D.’s, C.P.A.’s. Also use apostrophes for plurals formed from single letters: He gained A’s and B’s on his report card. Mind your p’s and q’s. There doesn't seem to be any consensus on this in https://tetrabookmarks.com/story20005309/details-fiction-and-car-finder-uk