
Part of the post has broken off the die on the mint mark area. The post forms the center of the mint mark. When it breaks off it may look shallow like (1979) on your coin. When the post breaks off at the base of the die, then it can actually appear higher than the mint mark (1978). This is often called a filled mint mark, but that is not the case. The mint mark post broke off the die, so all coins after this happens will show the same issue sometimes for hundreds of thousand strikes after this happens. So it is not a rare even.
It is common.
Edited by coop
03/26/2018 3:00 pm