I would say lamination. It appears as if the "piece of laminate" was recently removed giving the different color underneth. IMO
It could be a lamination issue and the top layer peeled away as suggested by fishaddict it could also be postmint damage of some sort. While the photos are clear it is always hard to tell unless the coin is in hand...
I don't think of lamination problems occurring before 1982. Hey, it's just me but I think of them as planchet voids. Could it be something like that?
planchet voids- This is a term I have never heard of. Definition or description would be appreciated.
Lamination errors occurred long before '82... Here's a 1959 DDO-002 with a lamination peel in WE: From CONECA: Lamination crack: These planchets have pieces of metal “peeling” from them. The lamination can be completely missing or retained on the planchet. The larger the lamination, the greater the rarity. Laminations are frequent on the 35% silver war nickels of 1942-1945. They are common on wheat cents, but are rather rare on 90% silver coinage.
Most likely not a real term; just a portion of the planchet, void of planchet such that when placed inside the hub the imprinting had nothing to be made from. But I went back and looked again of the ops photo. It would not explain their example. I have some pre '82's that resemble jcuve's most excellent photo #4 example. It's just that when a novice, like myself, 1st sees them, one thinks "Geeze, there is a part of the impression missing and it could be because the planchet was thinner there".
In any case, the coin at the top of the thread is simply damaged. Look at all the marks on the rim of the coin. It was just banged up , that's all. Fresh metal has been more recently exposed or the metal that is lighter would have toned along with the rest of the coin. It can't be a flaw in the planchet before the coin was struck. Thanks, Bill