CANNON PLUG, FUEL OR OIL TRANSDUCER
These Factory New Russian Cannon style plugs are how both ends of the Transducer cables terminate.
On the Transducer itself at one end ( the same plug for Oil and Fuel) and at the Tri Gauge at the other, behind the instrument panel.
Failing solder joints are the most common, (after forty plus years of faithful service), and erratic Tri Gauge indications will be a clue that something is amiss. We recommend you check the instrument panel end first.