Unfortunately, errors seem to reduce the value of key-date coins. Date collectors don't want a "defective" coin, and error collectors don't want to pay a key-date premium. That said, I'm pretty sure the price on that holder is old. If the coin hasn't been cleaned or otherwise altered, a low-MS 1921 goes for quite a few hundreds of dollars.
You sure?to me it looks heavily polished and the area in question is damage from the process.Its not near MS,plenty of wear and or damaged,I'd give it a AU 53 details.
It looks like a strike through when I compare it to TPG identified ones to me. It also looks slightly polished. I see no real signs of metal forced to one side or the other.
1921 Peace dollars didn't always strike up that well due to the high relief. If you're referring to the hair over Liberty's ear in the center of the coin, what you see is NORMAL for a Mint State 1921 Peace.
There's an artificial veneer on it I'm seeing in these pics. It looks treated with something. Possibly blending in that PMD, that'd be one rationale.