Is the term PoWs (POWs) inconsistent with typical acronym pluralization? [duplicate]
Solution 1:
No, the way it works for POW is typical. One pluralises the initialism rather than forming a new (but the same) initialism out of the plural.
A person might have studied for multiple MAs, a conference might be attended by many CEOs and MDs, and so on.
EDIT: originally I said (based on what's in the OP) that RBI is the one that's less usual in that it is an initialism formed from a plural, and hence one does not pluralise the initialism. Apparently that's incorrect, RBI stands for 'Run Batted In' and RBIs is the normal plural form.