Though I'm not an expert. It looks like a retain cud to me. Send it in to cuds on coins, you don't have to send the coin in just send the photos in two Mr. Neff, email with the photos. Most of the time if he's not sure he'll request you to send it to him. He is generally pretty good about replying back to his emails.
I would definitely send that one in to cuds on coins. Email all four photos to Mr. Neff. Like super Dave said it is a nice one,
Like I said not an expert. But from what I can see by the additional photos. The crack runs down both sides of the rim. And if one thing I've learned on CT. If not sure yourself send it in.
CU-1c-ND-51(Zn)(198X) ??? http://cuds-on-coins.com/wp-content/uploads/2013/09/IMG_8533.jpg http://cuds-on-coins.com/wp-content/uploads/2013/09/IMG_8534.jpg