That switch is definitely custom made for Kustom. The stop mechanism is usually just behind the threaded mounting section. If you look at the back of the mounting plate, you might be able to see if the rotating limit mechanism is out of alignment with the end stop blocks.
As for the hum, a grounded cord is always safer than the two wire ones. All of the preamp boards are grounded by the pot mounting hardware, so sometimes just removing and reinstalling the boards cleans up the ground connections and fixes minor hum issues.