EF-40 I would give it a soak in acetone to try to remove a lot of that crud on reverse . Now that I look at it again this coin was probably cleaned and retoned .Hence Adams comment about color being off . I would send it back to seller if you could .
Give a man a fish and you'll feed him for a day . Teach a man to fish and you've fed him for a lifetime .
The color of the coin looks a little unnatural. It could be due to cleaning, or the lighting when the photo was taken. I was guessing the later when I made the purchase. I will examine the coin carefully and provide my own photos when I receive it.
Usually acetone is a solvent that, most of the time, will not damage a coin, although on copper coins, it can affect the color of the oxidized top layer. E$specially if there is something on the coin like a transparent layer of glue that gets dissolved and exposes the surface to oxygen and oxidizes the coin. For many coin collectors, soaking the coin in acetone is not considered "cleaning."