Looks like a thread ( no pun intended ) , is it in the Coin ? It could also be a piece of wire like potty said .
The two im thinking are strike through wire or somehow someone's hair landed on the coin and it got struck into the coin.
if you zoom in closer whats that small line above the four to the right near the rim a clue?it doesn't look like md
The streaks on the obverse sort of look like the marks from a paint brush. Like light streaks of glue.
Possibly a hair as stated? I have a friend who said he found his first grey pubic hair. It was on his Big Mac....
Looks like a struck through to me. Like a small piece of wire. Definitely not retained but a lot of Strike throughs are not retained. Here's one of mine: