Every spanish book I’ve ever seen, and every map printed in spanish abbreviates “Estados Unidos” EE.UU.
Always with 2 letters. I’ve never been able to figure out why. Anyone know?
Plural. 
This page (third question) claims that double-letter plural abbreviations, like ‘pp.’ for ‘pages’, come from Latin. As to why the Romans did that, I never took Latin, but isn’t it true that there was no common plural ending, like the English ‘s’? So, instead, if you’re talking about more than one object, you just use more than one letter.