I have cleaned up four successfully so far, but avoid the temptation to take them apart too far. I've made that mistake on one. They are pretty simple but easy to break/shear screws/lose tiny parts/get things out of order. Opening up at least the body might be necessary just to clean them up. And keep in mind the pot metal these are made from does not flex.
Up where I am, spiders of all darn things go inside them and make nests, creating a lot of corrosion. Has been possible to rescue the above four, but until I opened them up there was no way to tell what was hiding inside. Just looking into the horn flare (?) they "looked fine" but didn't work. A good cleaning and some patience, they worked - play with that adjustment screw for sure. And yes, as noted above, 6V horn works on 12V just loud as blazes.
I think I still have one more on the "queue" to look at one of these days.
