STI stands for Shimano Total Integration, this means brakes and shifters mounted together as a single unit, IE integrated. Indexed downtube shifters are indexed downtube shifters. STI for a road bike at least, means you are shifting and braking in the same place. Now SIS, is correct for indexed downtube shifters. It just gets really confusing when you call a downtube shifter and STI, because it isn't. Finding six speed indexed Shimano parts is a needle in a haystack kind of find. It was VERY short lived, as the 7 speed norm was coming up fast on it's heels when it was hitting the sales floor.
Edit:someone already beat me to it, oh well
75 Raleigh Competition, 76 Raleigh Professional, UNKNOWN year possible Geoffrey Butler, 86 Raleigh Super Course. 71 Raleigh Super Course, 1980 Raleigh Super Course, 1974 Raleigh Super Course, 84 Club Fuji, 89 Team Miyata, 88 Schwinn black chrome High Sierra, 88 Cannondale SM1000, 79 Cabela, 1985 Custom pearl orange Schwinn Super Sport with full Sante group, 74 Volkscycle Mark 100 Full first gen Dura Ace, Early unknown year CW mountain bike with chrome frame and Campagnolo dropouts. (more later)