as it would be used in POW's

Here (and I'm winging it a little), the apostrophe is used because it's an abbreviation (actually an acronym), so the apostrophe is used for the contraction.

However, I believe that apostrophes on plural acronyms are falling out of favour, much as the dots between the letters have.

In particular, POWs would expand to "Prisoners of War", so why is the "s" at the end, and why the apostrophe?
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-- roger