More correctly "anti-loosening" plates. Given the number of loose rotors (maybe not coincidentally models that lack such plates & bolts) I see through the workshop, they're a good idea. Just note they need bolts to suit. There are two different sets that I know of; one has a little sticky-uppy tang on each end of the plate which bites into a toothy underside of the bolt heads, the other type has a couple of folded-up fences on each end of the plate, with triangular-headed bolts. You can run without them, but better with them. I probably have a few spares at work that I can send to you.