I found this meter on the side of a water tank in rural Essex. There must be a float in the tank that is connected to the red and white level indicator by a wire. The odd thing is that the higher the water level, the lower down you have to look for the reading. What is the relative weight of the float and the indicator? The float must be less dense than water but have enough mass to keep the wire taut. The indicator must have enough weight to keep the wire taut too. I'm guessing that they will be the same weight. If the indicator were heavier than the float, the float would stay suspended when the water level falls.
