Yep, that's fine - that's the exact one that most people have! There now apparently is an "ES" model as well, and there used to be another one with different letters, but the MS PLUS is the one that's the most widely sold.

Hi. I currently have a Casio fx-570MS.
I've checked online and the only minor difference is that it uses a button-type battery instead of the two way power that fx-991 has. So it doesn't have the little solar panel on the top right and looks different that way.
Is that too obvious and not acceptable?
I think McMaster should accept all calculators with versions fx-991 or lower rather than just fx-991, since it's quite obvious that having calculators with lower capabilities won't give students any advantages during exams.

To be on the safe side, just go with the fx-991ms or the one that titles sells, some exam I have written in, do check the calculators.