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If there is more than one letter or number in a term like that, I do not use an apostrophe -- MRIs, EKGs, systolic in the high 140s, etc.
If it just one letter or number, I use an apostrophe -- serial 7's, filed under the M's, etc.
I have no reference telling me to do this .. or not do it.. it is my personal preference and what I think looks good. Never a complaint or correction.