M3/BN131 is a metric size, hex head cap screw made by Bossard, a specialty fastener manufacturer. It is made out of a medium-strength steel, with a black oxide finish which helps to protect it from corrosion. The M3 size of the screw has a 3mm outer diameter, the BN131 head has a hexagonal shape, with a diameter of 5.3mm, and the length of the screw ranges from 7mm to 250mm. The screw is typically used in the automotive, electrical, and industrial sectors.