Probably Struck Through Grease issue. Compacted debris didn't allow the S to form properly when the Die struck the blank planchet. Nothing major.
Agree that it's a debris filled die error. These are considered minor and rarely add extra value, especially when circulation wear and tear is starting to make the coin look rough. Personally, I think these things are cool but I would probably spend a coin like yours due to condition. Nothing wrong with keeping it if you like. After all, it is a mint error. And no shame if gets thrown in the spare change jar.
If you like these kinds of finds and think they are cool, then throw it in a 2x2 and put it in an album, because it makes YOU happy. Just leave a note for the heirs - "When I'm gone and you find this, it's a book of errors with no value beyond face value that I kept because they made me happy. Don't expect you can buy a boat of any kind after you sell these."