It has a nice toning on the reverse and did on the obverse, but some sort of chemical took it to the bare copper.
Nice find! I’m in the Elmers glue camp. If it is just Elmers you can soak it in acetone and it will remove it. If it’s the school type, a water soak will loosen it.
If you look in the third picture you can probably see the split serifs from the tilted RPM. Acetone or water is a good idea.
Thanks for the pic. I still think the quality of the pic, leaves guys like me squinting and bobbing our heads.
Fair enough! I think it is more of the glare that can throw people off. It only looks a little blurred too me, but that is just imo. Also, looking up the variety can help when verifying where to look. Hopefully this link works: http://www.coppercoins.com/lincoln/diestate.php?date=1957&die_id=1957d1mm017&die_state=mds I have not looked at all the RPMs for this date, but I trust the OP's attribution.
Here's another attempt at the mint mark, when I crop the pic, it looses focus. Also, a pic of the stain, coin was soaked in acetone, it is not Elmer's. Thanks for all the interest.