99 is 2001. I have a 98 myself. I bought it new in 2002. 1991 would be 09. You subtract from 100. The S is sport. It was made from 81 to 84 according to the Brophy book. After that came the SS. Interesting. You have an older manual.
Marlin redesigned the 1895 with a pistolgrip. The S was the first reference for that.