You've done well! (rightfully could have paid $400 - $500 more and still have gotten a good-to-you deal).
Could you tell use the code on the muzzle end of the barrel? Does it say anything like C MP CHROME BORE, or C MP C, or even just the number 12?
If you take the screw out of the front pivot pin, is the hole in the upper .250" or is it larger like the receiver?
Under the sight wheel on the ejection port side of the upper, are there any letters like C K, or is it blank?
Can you post a picture of the bottom left corner of the dust cover where the upper meets the lower?
Along those lines, could you take a close up picture of the area to the top right of the dust cover? There may be some faint stampings in the metal.
I slightly suspect that the upper may not be original SP 1, but looks could be an old military upper.