two disc mounts as nobody had settled on a standard for mounting them. what we now know as I.S. was maguras offering from 1999 i think. and post mount is what hayes were offering in 2000, as the earlier offering wasnt any good for suspension frames with the seat stays and chainstays close together.
twin discs were offered in 1997 by cullimore engineering (C.H.E.)in the uk, test rode some at the malvern classic, VERY powerfull, almost too much power. very nicely machined.
oh, what does sts stand for, i think rts is rocker travel system, lts is linkage travel system, not sure tho?