You must live in a region where leaded fuel is still sold, therefore your car does not have an O2 sensor nor catalytic converter. That part is your fuel mixture adjustment trim-pot, because the ECU cannot rely on feedback information from the O2 sensor to adjust the air/fuel ratio on the go. On older models this is usually mounted to the firewall or near the throttle body somewhere. Basically I would just leave it alone unless your car is obviously running too rich or too lean.