1964 Jefferson (Example Of Die Attrition?)

I would think that it is a combination of miniscule metal dust particles and grease compacted along the edge of the reverse die, which is on the bottom of the press and the obverse is the striking die. It would make sense to me that gravity and the reverse die being stationary would gather more than the moving obverse die.
The hard grease and metal dust become part of the design unless it is cleaned off the dies over time.

This does look like one of those "black beauty" nickels though. I would value it more for that personally.
http://goccf.com/t/288744

Looks like extra metal...like a folded Rim Fin.

Thank you all as always, and I definitely have some more reading to do.