1978 Quarter Supposed To Br D Mint Mark I Think But Not Sure, Need Help

When your coin was struck, a part of the die that would form the inside part of the mint mark broke off the die. It is called a post. The post and other forms of this creates the inside portion of certain devices. (B, D, O, 0, 8, 9) when this post breaks off, then the area inside the die is filled on the coin. Why? Because now there is a void in that area of the die. So nothing forms there except what you have. Sometimes the post can break off half way, split down the center or even bread down below the post on the die. Not that area raises higher than the mint mark. But no premium for this. Just knowing what caused it is your reward.