The 84 EVTM shows the overboost light has a dual function.
Beside being turned on by the turbo pressure switch, it is also turned on by the "direct mileage sensor".
From the How it works write-up:
The Direct Mileage Sensor turns on at approximately 30,000 miles and turns on the Overboost Warning/ Check Oxygen Sensor Indicator, indicating the oxygen sensor is to be checked.
The component location list doesn't show a location for the Mileage Sensor.
The 84 Owners Guide doesn't mention a Check Oxygen Sensor light.
It disappears from the 85 EVTM.
Never heard of it either....