This a rare hard to find die cap that stuck to the hammer die and eventually formed a high rim in the shape of a bottle cap. As it was finally ejected, something bent a portion of the rim. It could have been caused by the mint employee trying to pry the cap off the hammer die. However that would leave more pronounced gouge marks and scratches which are not found. The photo looking down into the gap of the bent portion shows a smooth but curved bottom. My thought is that a coin happened to sit upright vertically and was pushed down and into the rim of the cap by the hammer die. That would mean the second coin would be an edge strike or be bent too like a curved coin. See similar condition at last two photos. I happen to have a bent cent, and I placed it in the gap and it stood straight up. I'm not saying the coin I have is the one that matches, but in my opinion, this is very close to what a mated pair would look like. The bent coin is sold separately and can be found under CENT errors.