Looks more like Environmental Damage I metal detect and have found dozens of Nickels such as yours Not a Mint Error.. Worth 5 Cents This is due to the nickel being exposed to dirt, sand, soil, the weather elements. Also occurs on Clad Coins.
well, (a) get a propane torch canister and nozzle (b) turn on the gas (c) light the gas (d) point it to a coin you've now torched a coin. but it looks like environmental damage anyways.