Everybody is correct and everybody is wrong.

According to Turabian's "A Manual for Writers", plurals of most single
and multiple capital letters used as nouns are formed by adding "s"
alone. Example: SWRs
Form the plural of small letters, capital letters with periods, and
capital letters that would be confusing if "s" alone were added by
adding apostrophe and "s".

Now, everybody, back to work and mind your p's and Qs

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