They've nailed it. A PCGS 4 slab. Green label made with inks that were not color stable. Usually the blue breaks down and you see shades of yellow, but the yellow can also break down giving you blues up to and including the blue used for the labels starting in 1999. I've even seen one where the top half of the label was blue and the bottom half was still green. At one time I had a collection of 13 labels from PCGS 4 slabs, each one a different color.
And I love the color on the obverse. I would have bid higher if these were the pics. Too bad he is now asking more than $100 more than what it sold for last time.