R34 Fuel Tank Gauge

Our R34 (built 1972) has a stainless fuel tank with a direct reading, side mounted gauge.  The gauge dial is visible from the aft, inboard end of the pilot berth; just above the berth boards and under the throttle control.  We suspect the gauge isnt working properly.  Perhaps a flooded float inside the tank.

Do any other R34 owners have this type of gauge fitted.  We don't know if it was a factory installed item, or added by a subsequent owner.  Any reccomendations for spares or direct replacement (or complete alternatives) welcome.