1879-S Filled S Need Help

It,s kind of hard to say. It isn't PMD from what I can tell. Very interesting

Interesting thought there Dad, I would expect to see die cracks still visible above and below the S, I'm leaning on PMD earlier in its life, and worn flat, much like some known knife cuts that have been submitted as large gouges......

I'm guessing you accidentally hit "New Thread" instead of posting those images here, so I made that one go away.

This is a C Reverse, called "Reverse of 1879" or "third reverse," not a Long Nock, which has a parallel top arrow fletch and not the angled one seen here.

My guess is it's a sliver of extraneous metal which got caught in the strike, and squeezed onto the coin permanently by 150 tons of pressure.