Great price on the OP-addition (congrats) Hmmm, but that circle does look a bit odd, eh? (it isn't nearly as noticeable, or curious looking in the seller's pics) .... obviously, I hope that your coin is a winner and that the circle is merely a blemish, or something else that's explainable
The blob looks like someone filled in a banker's mark or punch hole and it is has since separated around the edges.
Looks like the 2nd loop of hair to me. Poorly done by the engraver in high relief and then worn down.
Since I paid about $200.00 for my example, I think you did very well for the type/grade. I alo think TIF guessed right about that odd circle....and please liberate Athena as soon as possible