One way to distinguish the counterfeits from a genuine 1931-S are the position of the "3" and last "1" in the date and of the "3" relative to the mint mark.
The line across the top of the first and last "1" should cut off the top of the "3." The line across the bottom of the first and last "1" should only clip a small portion of the bottom curl of the "3." The line connecting the top and bottom ends of the curls of the "3" should cut through the top serif of the mint mark. A diagonal line from the bottom of the first "1" touching the end inside curl of the "9" should intersect the middle bar of the "3."
Here is a known genuine 1931-S (from NGC's site):

Here is your coin, with the same marking lines:

Even though your photo is smaller, the lines match up well. I agree with the upthread comments, VF details, environmental damage and cleaning.