The 79D mintmark was called "the dot" because the punch was so overused it was damaged on the inside post(broke off) and the lower left corner was broke off too. What you have is a contact hit on "the dot"...LOL!
If it were an over mint mark, the 'S' would be hidden under/behind the mint mark:
But on your coin, it is not like that at all. There was a contact mark from another coin/something else that altered the mint mark. In 1979 the only 'S' mint mark used, then were on proof coins. So this is just PSD.