Possibly a die gouge. I haven't checked to see if there is any possibility of it being a clash, but it is most likely one or the other.
That mark if you get a flashlight out and and set the Quarter on the table and then rotate the light around you will eventually see the mark plain as day.. I had to look for a while before I figured out the right angle that the light had to be to see what it is.. Its a upside down slightly rotated number 6. I also found another one to. If you leave the Quarter on the table and move the flashlight up next to the O in you will find a hidden 7.. Or thats whats on mine.. They would all be the same right? Can someone double check me on that.. Wanna make sure what Im seeing is right..
I posted this almost three years ago... when I saw your response, I took another look at the quarter. What I now believe I have is a die clash 2007 IDAHO quarter. I will post the images below. thank you countrygurl...