Environmental damage of some sort, most likely. Or possibly some intentional damage using one of many chemicals. Or possibly a plated cent that was then damaged. Either way, its just post-mint damage.
The zinc in these pennies just can't stand up to the enviroment. 1,000 years from now, numismatist of the era will believe that the USA stopped miniting pennies in 1982 because they can't find of trace of any after that date.
Zincoln........copper coated zinc. The copper has worn away due to environmental issues, and the zinc core is showing.
