1979 Filled 'D'?

The S mintmark punch was changed roughly mid-year. The two different styles of S mintmark "appear" to be filled or clear, which is why we call them that.

What you have is a normal D mintmark (all were the same size) that had a broken center, thus the center wasn't punched. It is considered common and not worth a premium value. Because the difference is caused by a die chip and not a completely different letter style, it is not mentioned in any guides.