Needs a combination of adverse environmental factors for the 200 parts fine copper to produce the green spots you see here.
Factors in this case could be:
-included gunk, - providing some aggressive chemistry
-moisture trapped in the holder,- acts as a chemical transport medium at the spotting sites and takes part in green salt formation, and
-200 parts fine copper.
If the holder is acrylic plastic, that is not the cause. Pictures suggest hard acrylic plastic.
In most environmental circumstances, this sort of green spotting will not occur, even if only 0.800 fine silver.
Give it a good acetone soak, and with bit of luck, the green spotting will disappear, hopefully without leaving any obviously visual sort of damage.
I find it impossible to believe that PCGS
would not be obliged to comment on the green spotting, if present at the time of grading, so I must conclude that moisture and gunk was not visually obvious.
If it is not taken out of the holder, I suspect that the green spotting will only get worse.
Excellent pictures, but grade seems closer to MS-62, to my eye. Obverse better.