If I'm not mistaken, the 84 Centurion Comp TA has the same Shimano proprietary shifter bosses as the the 85 Centurion Ironman (and other period bikes). These are rounded with a keyhole tab at the top of the boss, unlike the standard square bosses normally used. Additionally, they're threaded for M4.5 bolts instead of the normal M5 bolts.

versus


If you're going with brifters, finding shifters isn't an issue, but you'll need to modify a pair of shifter cable stops to fit these bosses.

This link explains how to do it The Kodiak saga continues | The Simplicity of Vintage Cycles

M4.5 fixing bolts are hard to get, but are still available at Loose Screws. Theses are Dura Ace Shifter screws with a D ring cover that will need to be removed (see picture at the Loose Screws link above). Just put it into a vise with the screw end facing upwards and gently tap it with a hammer to loosen it up, so you can remove everything from the screw. The screw may, or may not have to be cut down slightly depending on the frame bosses. On my 85 Ironman I had to remove about 1/8 inch from one to make it completely tighten down the cable stop to the shifter boss on the front derailleur side.

Use shifter cable stops that are as rounded as possible to provide the maximum amount of internal cable stop space when you remove some of the inner material to fit the cable stops. This will make it easier to get the cable stops to sit flush against the frame tubing. The ones I used had flat sides (like the sides of a pyramid) and can't fit completely flush against the frame tubing (about 1/16 of an inch gap).

If you decide to go with downtube shifters, Loose Screws also sells Shimano Z-401 downtube shifters that supposedly go with these weirdo shifter bosses. I haven't tried them myself.

If you try looking for other compatible Shimano shifters, the model numbers will have the suffix FAI, which should all fit these downtube Braze-On A-Type/M4.5 bosses.

You're other option would be to modify the bosses themselves by removing material from the bosses to jury-rig a fit for shifters that fit normal bosses. There is a thread here on this process at http://www.bikeforums.net/classic-vi...ter-style.html