Silver Washington

What does the tiny inscription JF, just below Washington’s head above the 4 on the 1964 silver quarter mean?

I have been looking up my coin collection values, and I know very little about them, and this caught my eye and I couldn’t find any postings anywhere about it, so I turned to you guys. It is extremely tiny and you’ll probably need a magnifying glass or hand lens to see it, but it’s just above the 4 to the right on the 1964 silver quarter. Also, does this make it more valuable?

Those are the initials of the coin designer, John Flanagan.

Michigan Treasure Hunting Silver Washington’s