For abbreviations – both acronyms and initialisms – where the last word in the abbreviation is the noun, I'm accustomed to adding an 's' with no apostrophe (e.g. ATMs), as described in the answer to this question: What is the correct way to pluralize an acronym?
But how do you pluralize abbreviations where the noun is not the last word? E.g. 'Point of Presence' (POP)
I can see how 'POPs' could read 'Point of Presences', when the intention is 'Points of Presence.'
Best Answer
Virtually always, the s follows the full abbreviation even when a word other than the last one is the proper plural. Paul Brians in Common Errors in English Usage cites
This style is also recommended in the AP Style Guide, so it is what you will see in most newspapers. But not all.
The other option is to discard the s altogether, as in this news story from the Cleveland-area News-Herald:
This is certainly not a typo. Many commentators prefer this, but it is simply that, a preference. As a former editor at ESPN, I had this discussion many times with writers and other editors, many insisting that there is only one right way to do it. Clearly, that is not true. If you prefer the s, use it, but only after the entire abbreviation. Nobody, but nobody, uses RsBI.