Thats odd. it looks like something after the mint, but usually there would be some signs of it on the obverse also.
It MAY be a planchet flaw. If an air bubble or something else is trapped in the coin. Normally it will spilt the planchet in half not just a piece out of the reverse. It is in serious doubt though because there looks to be more than a few scratches that run to the right of the area in question. Plus their are four pieces of coin left inside the area that would not be consistant with a planchet split. I think someone was trying out a new etching or rotory tool like a dremmel.
From squinting for a few minutes at your poor-quality photos, it looks like a lamination peel may have fallen off during the time the coin was circulating, causing a partially-split planchet... Although, as I said, it is very difficult to tell from the photos you've provided. -Brian
I don't think it is a lamination, surfaces in the depressed area seem too smooth. I'm thinking a strikethrough. possibly through part of the clad layer from a dime.
