Low Fuel (7L 4L) sensor stick/ball transition?

[schrocketeer]

When did the sensor that drives the 7 Liter and 4 Liter low fuel lights transition from the capacitance "stick" to the floating-ball-on-the-arm?
On my new to me '86 K100RT (WB105140XG0053944), it has the orange'ish ball on the arm in the tank. The 4L level light seems to be working consistently. I tried lifting the arm and letting it lower slowly to engage the 7L light, but I didn't see it come on. Understand that may be a bulb issue in the instrument cluster.

Is the ball/arm system more reliable than the capacitance stick?

[Laitch]

The float system replaced the thermistor system because it was more reliable. Furthermore, the two fuel level warning lights were abandoned in favor of one light only. The "choke" light was eventually abandoned also.

[rbm]

Is the ball/arm system more reliable than the capacitance stick?  <== Both are reliable if both are working. The float arm low fuel indication scheme is a more direct a way of turning on a light. 

The earlier gauges had two lights with two pins for inputs - 7L light used pin 8 for input and 4L light used pin 7 for input.
As Laitch points out, on later models, the 7L indicator was eliminated leaving only the 4L indicator. The float sensor low fuel sensor is wired to pin 7.  The sensor has two circuit cards - one with a wire-wound resistor to give a level indication and another card with copper conductors for low fuel indication. Normally the float arm is held high by the fuel level so there is no connection to illuminate the low fuel lamp in the gauge. When the fuel is low, the float arm makes contact with the copper traces on the second card and the light illuminates.