Metric Nylon insert flanged lock nuts have a nylon (usually a similar polymer and not actually nylon) ring inside of the nut. The are not always serrated as the nylon ring provides the locking mechanism. When you put the bolt through, it starts to thread the nylon, then it puts pressure on the screw and prevents it from vibrating loose or falling off. The nylon insert nut features an internal nylon ring, the addition of non-metal material makes the nut more sensitive to temperature and chemicals. These nuts should not be reused as the ring is damaged by design as part of the installation process.