grammatical number - What is the correct plural form of an abbreviation whose last word starts with 'S'?

What is the correct plural form of an abbreviation whose last word starts with 'S' (thus making the last letter in the abbreviation an 'S')?

The example I keep running into is "CMS", an abbreviation of "Content Management System". How would I refer to a group of these beasts?

Answer

I think CMSes is perfectly fine. Some would say CMS's is okay, though I prefer the version without an apostrophe.

I don't agree with CMSs because CMS is an initialism where each letter is pronounced, not an acronym like SCUBA. (Though others might have input on the words initialism and acronym, you know what I mean.)