Mint Marks were hand punched into the Obverse Die back then. They are located in different areas in that location. It's normal. @potty dollar 1878 You beat me to it! Good job
Aaahhh, much clearer now. I thought they must had to have a bunch of gremlins working 24/7 punching each individual coin and explains how I can find a plugged S. Thank you!
It belongs below the date, to the right of the bust and to the left of the rim. Anywhere in that space is acceptable.