I'm thinking damage. The edge on both sides has been worn down. I'm not sure what did it, but the culprit may be the same thing that has worn the center of the reverse.
Yeah, sorry Patrick. I would agree with Enamel in classifying that one as PMD. Not sure what would have caused the damage but my guess was that the coin was face down in something with the reverse exposed to constant scrapping. Being the Monticello sticks out higher than the letters it was the first to get scrapped off.