Late 1970s D mintmarks were frequently mushy and indistinct due to a worn mintmark punch, same situation with the S mint punch used for proofs. Your coin has also sustained a significant amount of PMD around the date and mintmark area.
The center post of the mint mark has broken off. Without that post the center post of the mint mark can no longer form the center of the mint mark. The rest is damage as biokemist6 mentioned. Note the damage to the date?
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