It's an "or what" Either a contact mark from circulation or the copper plating has been breached and the zinc is corroding under it. There is zinc rot in other places. Not an error of any kind
One post per coin is more than sufficient. Take people's word if they tell you something is not valuable. Otherwise there are grading services you can pay for and they will probably tell you the same thing.
I see the D you are referring to. That is not a mintmark. It looks nothing like the date or anything raised on a coin. If you look at something long enough you'll see things that really aren't there. Welcome to CT.