I think that might be a grease strike-through error, which is fairly common, but still cool to find. This happens when there is a little bit of grease from someone's fingers on the dies, and when they strike, the area affected by the grease does not produce as much detail as the other areas of the die that are not affected by the grease. Since it is quite easy to accidentally place a finger on the die while making it, these errors happen a lot of times. It is still a keeper. Some other strike-through errors include through hair or cloth or even staples, which are obviously rarer and consequently more valuable.
It might have also been dipped in some acidic substance considering the erosive lines in the fields and the redder color.
Looks like an over-polished die to me. You can clearly see die polishing lines all over the coin. Maybe some grease too.
