It may have been plated. I've ran across several that have been plated over the years in a variety of materials. I believe I still have some nickels and dimes around here that were copper plated. Honestly I'd recommend cleaning it. In its current condition it's worthless anyway. Or you could even weigh it, when anything is plated it will add weight. To clarify though if it is plated it was not done until AFTER it left the mint.
Copper, being a metal, will change in color depending upon what it's exposed to. Does steel rust if you leave it outside? Ever seen copper on houses that turns green / or bronze statues ? Copper, on the lower end, tarnishes. which is oxide formation with the copper, as when it's exposed to air. It can start turning pink at first, and as it gets "thicker" and is corrosion it then later turns black. at least that's my simple understanding of it. Ever seen a silver coin .. or anything silver, turn black over time ? Ever use Copper plated cooking ware ? Look up Copper Oxide .... it's black.
Black pennies laid in wet soil or rotting vegetation like leaves. Really old silver will turn black in salt water. Silver stays shiny in normal dirt. j
Air also has a lot to do with it. And what other things are in the dirt. In Detroit there is a oil processing plant that spews Sulphur. You an smell it, it's aweful. If you put a nice and shiny silver anything outside for a week it will probably be black in no time. Versus someplace much further away with better air quality. I'm sure my car corrodes some every time I drive past the place.
"Air also has a lot to do with it", says Clawcoins. But, surely, it isn't the natural oxygen ? it's, for instance, due to that Sulphur spewing place, which you can smell. As well as industrial, or other pollutants, such as car fumes !!!
I missed that it was pre-82. Yes, thank you for pointing that out. In that case and in view of verifiable facts. I stand corrected.
I had more than I thought... I threw a few random pocket coins in just for contrast. As you can see there is no Rhyme or Reason to it, someone just randomly plated them in copper I'm assuming only because they could.
Not plated.. Just environmental damage.. A few weeks ago I put hundreds of them into a Coinstar machine. You probably got them from my batch. They were all Metal Detecting finds.
Just saw the photos from @Curious Coin and they all look like environmental damaged. I see no signs of plating. If you keep them, there will be that many fewer error coins in circulation.