Pack of 2 - M10 x 1.5p Serrated Flanged Hexagon Nut in Stainless Steel.

These are an anti-vibration nut which use their serrations to bite into the mating face and prevent them coming loose. It is important to remember the face of the component they mate with will sustain damage. Do NOT use with a plain washer as this will defeat the object as it will only lock itself to the washer.