Postscript to the story:
Roberto came back an SGC 6.5, so they did, in fact, stay true to their word and use a loupe (see post #7) to closely investigate the area and determined, "You bet yer sweet butt that ain't no stain."
When I originally bought it, my brain told me it was nothing short of a 6 with that pretty centering (this is always a tough card to find decently centered), so the grade matched what it was supposed to be (for once). Of course, the black of the tuxedo slab kind of makes the corners look worse than when it was on a 'normal' background, but that's life.
1969clemente50SGC6.5.jpg