There are small ring-shaped craters all over this coin - notably, there's a string of three or four them coming out of Lincoln's eyes, looking almost like a cartoon thought-bubble. So the similarly O-shaped crater below the date is likely to be caused by the same thing that caused the others. That it just happened to form where a mintmark would normally be stamped is a coincidence. It was probably originally an un-mintmarked (Philly mint) coin.
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On post strike damage, wouldn't there be evidence on the other side like some type of indent or anything?
That depends on what caused the damage. Getting hit with a large pointy object like a hammer-and-nail certainly would cause reverse damage; corrosion caused by fire or chemical damage would not. And this coin shows every sign of severe environmental damage, such as a fire or corrosive environment.
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