The brass alloy thread corrodes and gets stuck in the threaded receiver. The thumb screws can break if you force it.
If the plastic threaded receiver breaks or strips you lose the ability to aim the reflector / lens assembly. I've used motor oil and let it creep in. These may be an embedded in the plastic brass receiver and so corrosion form from exposure and they can seize.
The FAQ:
https://www.brickboard.com/FAQ/700-900/ElectricalLightingHorns.htm#Headlight_Aiming
Some liquid oil where the adjuster threaded shaft meets the threaded receiver.
A little unsure.
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