I believe it is the "Filled" 'S' http://www.pcgscoinfacts.com/UserImages/1981s Half Dollar Mintmark Comparison large.jpg Photo courtesy of PCGS
The first or Type I S in 1981 is the same as the second or Type II S in 1979. There is a slight difference in the shape of the serifs, centers, and curvature of the type I and II S in 1981. But it is subtle and nowhere near as obvious as the Type I and Type II S's in 1979.
