The definition of "mid", "mid-plus", etc., has changed over the years (as I'm sure you know) and Prince at one point did call the 90 (which was really a 93) a "mid-plus". There's never been a POG that would be called a MP now, though.
OP, that racquet did appear first at the time of the POG3 described above. I don't know enough about it to describe versions and subversions*, but I'd expect production changes of the Series 110 and Series 90 frames to be parallel.
I found and saved a thread here on TT that described four versions of the four-stripe (POG4) rather than two. If no one posts a link soon, I'll get the info off my other computer and post it.
* Actually, almost every attempt to capitalize on the Graphite reputation in a more modern line (Graphite II, Triple Threat, NXG, EX03) has been accused of "subverting" the good name of the Prince Graphite, but I guess that's not the same thing, is it?
Can you post the link of the 4 versions of the 4 stripes?
I Just got another 4 stripes POG 90 from a friend, but it is way "beefier" and the beam is slightly thicker than my existing pair (as thick as the EXO graphite mid, I think), so would like more info.