Looks to me like a combination of environmental damage and circulation wear. Wait for other comments!!
A good acetone bath should do it. There's a really nice old copper cent hiding behind all that grunge. Z
The whole coin tends to tone similarly, but the high points come on contact with flat surfaces and wear the toning off so they are lighter. If acetone doesn't change the color from the chocolate, just enjoy the highlights after all it was 70 years Jim
Well it's environmental, my thoughts on it's appearance is it was stored improperly, then someone thought it would be a good idea to give it a soak in used motor oil and the carbon in the used oil deposited on the surfaces while it also did help with the crusty that was probably there from storing it badly like in a cup holder of a car for a couple years. It's an auto shop trick to get the crusty off of cupholder coins. any oil will work though and the cleaner the better as far as how it look after a quick soak, but if you miss one or two in the oil pan, it goes into the used oil barrel, and eventually it gets pumped off and someone goes through the bits at the bottom and says "score I made 50 cents!" tosses them in the parts washer, pats em dry with a shop rag and then buys a can of soda out of the machine. I know, it's an elaborate story. I've been that guy and it was common in the collision shop I worked at when I was much younger, free soda from the machine! The customers really don't want their crusty coins (never once heard a complaint in 2 years there, all the techs did it) and they all preferred you give them a nice clean car back with interior detail after you're finished working on it. Never taking like bills or valuables of course (that never happened beyond coins being there) and never taking their trunk junk they forgot about but we'd organize it in an orderly fashion so that would be neat too. But the loose crusty coins were fair game.
The highlights on the coin are just the environmental toning being rubbed off, much like a bronze statue that people constantly rub.