https://www.circuits-diy.com/crowbar-overvoltage-protection-circuit-2/   A crowbar circuit is probably all you need for over voltage protection. It basically shorts the input to ground and blows the fuse. Crude, but effective. Just need to be realistic with the voltage you set it to so it doesn't go off in error.  

Quick post showing off my Recon Scout model. I've used various sources including the approximate proportions from the Combat Manual and a (very rough) 3D scan I did of the Character Options Figure, then used the Ref Gallery and eyeballed it from there. Still needs a bit more work and the textures are just placeholders for the moment.  

It's interesting that the thread you posted implies that it was new in Series 3, but I found one above that was all the way back in Parting of The Ways, unless I'm mistaken. Now that I'm looking at it again, I think it might be a detail on all of the props (at least the early ones), and the one I posted that's missing that cover is actually just hidden behind the eye disks. Edit: I just added the pulley cover to my model to check, and sure enough, depending on the angle, it does disappear completely behind the eye rings.

I believe it's a cover for a pulley and drive belt. Most likely bigger components than those used on the first prop.   There is some discussion about it here: