The same impact that caused the rightmost rim gouge moved metal up inside the N. It's a damaged coin, not an error, but I'd certainly keep it -- I'd probably pull it out to settle arguments. Welcome to CoinTalk!
Sorry, but it's only damage. The letters aren't added to individual dies, they are on the master hub. * The master hub is used to make master dies, * which are used to make working hubs, * which are used to make working dies, *which strike coins. If a wrong letter was used on the master hub, every coin from that year (and other years since the master hub wasn't updated often) would have the same error
It took a hit and caused the soft copper to move, thus forming an M instead of the minted N. A neat find, one which I would keep.
