I'd call this a cracked planchet. A real mint error. Not a damaged coin issue. Nice find. Too bad it took so long before it was pulled from circulation. The value would have been a lot higher, when it was BU. So accept it as it is, and don't alter the coin. Altering may make a buyer doubt it is genuine. So, leave it as it is.
Man, that is cool! At first glance of the obverse I was like "boy.... that looks like zinc rot...." then I was like dang... that's a 59' copper.... then the edge is really cool with the split! If I had seen this one in a bank roll right off the bat I would have thought rot until I saw the date (with my magnifying glass, loupe and readers..lol). Just goes to show that we need to pay close attention to all the details on each coin. You definitely have some very cool errors you've posted here KeepTheChange, thanks. Very Nice!