That FS is on a trillion nickels. Give or take 6 or 7. There are some nickels, with superb strikes and excellent condition where you are able to see all (or most) of the separation between the steps on the reverse. The FS stands for full steps. (rare) It's just a coincidence that the designer of this coin was Felix Schlag.
The FS designation often used for Jefferson nickels means full-steps which refers to the steps to Monticello. The better the strike and the better the handling of the coin, the more steps will be seen.
Ohhhhh....I didn't know that....I thought FS was the designer's initials ( DUUUHH ) Thanks Guys for clearing this up for me!