This is the stock Mitsubishi Base Idle Set Screw, sometimes called BISS. It is located on the lower/back of the throttle body, and serves to mechanically set the throttle plate's base "closed" position.
The screw itself does not typically get adjusted, but if it gets loose, it can slowly vibrate itself out and end up missing.
These are the same for all 3000GT/Stealth/GTO.
Screw and nut sold individually.