Well, that's not the sort of thing I expect to find under a "PMD or mint error" thread title. Are those dark depressed-looking spots actually depressions? Can you see any sign of friction or dragging within them, if so?
On silver coins, I'd instantly call that "improper alloy mixing." I've seen it many times. But I've never seen it on gold before. Is it raised or depressed into the coin?
Coin is not in hand yet. I will let you know what it looks like in hand when it arrives. Insider, with ICG, said the same thing. Said he does not see it often at all (and never this large) on gold.
Rubber band mark would not engrave the gold, just leave a stain. If you blow up the pictures, the gold is actually deformed here.
I hate to dis a nice coin, but it does appear a small gear wheel imprint, but I would have no idea why, unless some type of antique counting machine. I can see striations which might be the groves in a milling bit. I do not think it is bad mixing of metal. IMO, Jim
Might be embedded grease struck into the coin. That actually makes more sense than improper alloy mix.
Your photos make it look like the surface of the coin is burned as a hot gear slid over the surface. I love the Quarter Eagle Indians.