Weathertron Thermostat Indicator Bulbs?

One of the GE Weathertron Model 3AAT80B1A1 thermostats in our home is doing its thing and if the temperature setting is increased a few degrees above ambient the auxillary heaters come on OK to supplement the heat pump's output.
But the blue indicator lamp has stopped lighting in that condition.
I suspect it's just a "burned out bulb". The thermostat is old enough so I'd doubt that the indicators are LEDs.
Before I attack it, can someone tell me if replacement lamps are available and easily screwed or snapped in or will I have to figure out what bulb to get and use my electronic repair skills to solder it in.
Thanks guys,
Jeffry Wisnia
(W1BSV + Brass Rat \'57 EE)
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