Hi, I'd say it's correct to use the apostrophe, because it signifies that there are (lots of) letters omitted. Best wishes, Clive
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Cases which end in a vowel (POs, BAs, CEOs, OBEs, DAs, NCOs) seem to him to resemble a tortured and unnatural word, not only an offence against nature but an abomination. He is, if you will, a victim of his own highly-developed aesthetic sensibility.
Case dismissed on grounds of compassion.