Hello JP what color is your Sport SX ande what Componets? I know my 1987 was black whith red decals and had Suntour Alpha 5000 componets, it also came in red. almost all Bianchis had the 86-87 World Champion sticker on them even my 1990 Proto frame set still touted that fact. I have a collection of catalogs going back to 1980 I'll try look your bike up if I can.

OH if it does not have index shifting it is older than 1986 or so.