Your frame was built during weeks 25-26 of 1990. That's a bit early to be a 1991 but I wouldn't rule it out. They may have been building frames to stock, as the carbon frames took longer to build. The 980C was spec'd with Shimano 105SC in both 1990 and 1991, at least in the USA. Shimano RSX wasn't released until the 1995 model year, so it's an obvious rebuild or maybe built from a NOS frameset.