For posterity's sake, mid-1990s sounds correct, as Tange introduced their series of Ultimate tubesets circa 1993. They were basically a refined version of Prestige with a slightly revised alloy and more advanced heat treatment, which allowed for even thinner wall thickness and lighter weight. While there was a factory ribbed version, this is not it. That is standard Gilco profiling, which increases the stiffness of the very thin tubes. Attached is the relevant page of the 1994 Colnago catalogue, mentioning Tange Ultimate Superlight as OEM specification for this model.