Die chip in the 9, common for the 50's, but your's is a nice one !! Is that a chip on the forehead as well? Edit: WELCOME to the forum !
Looks like a die chip in the 9. I believe grease would fill in the 9 on the die causing a weak or no 9 on the coin. I can't tell from the photo if that mark by the hairline is raised or indented. Bruce
Welcome to the forum, as jack said there is a die chip in the nine and as you search through more lincolns you will find some years that are good for this. One thing you might want to look at is the S mint mark, there are 30 different repunched mint marks for that year. Again welcome to the forum and good luck searching. BILL