Sometimes an apostrophe is used to separate the plural suffix from a base with letters, numbers (usually dates), abbreviations and such like. So you'll find "Mind your p's and q's", "1960's, "80's", "Ph.D.'s" and so on.
The books tell us that this practice is less common than it used to be; apparently with dates and abbreviations ending with an upper case letter, the form without the apostrophe is now more usual: "in the 1960s", "the 80s", "Ph.D.s"
I did a quick corpus check and sure enough, dates without apostrophes are indeed more common.