Who is IGS? I'm not big on TPGs but it's not one I've heard of. They don't seem to be that great with the grading IMO. All those blotches and stuff.
I don't collect slabbed coins and know very little about grading, but I would expect a proof to have better centering. Or is the centering an illusion?
I think it's an optical delusion. The holder is hiding the rim. My guess is that the spots came years after being slabbed. I wonder how it appeared back in the day.
I saw a 1922 Buffalo nickel graded MS70 by them. Just kidding, but I wouldn't buy any thing slabbed by them for their grade. "Buy the coin, not the slab."