What does "MP" stand for in the postal code of the Northern Mariana Islands?

Question

California is "CA." That makes sense.

New Jersey is "NJ." That also makes sense

Mariana Islands is...MP? I dunno...

I know it really doesn't matter, even to people living in or sending mail to the Marianas, but I've searched high and low for the answer and can't find it on my own. It's killing me. I get the "M" for Mariana, but where the heck is that "P" coming from? I'd prefer actual answers to c**p that you just think up off the top of your head, but if it makes me laugh, I guess i'll accept that too.

Answer 1

Pacific

Answer 2

P is Pacific.

They can't use MA, MI, or NM.  They are already taken.