M3 Countersunk Range DIN 7991 ~ Socket Countersunk A2 Grade M4 Countersunk Range DIN 7991 ~ Socket Countersunk A2 Grade Socket Countersunk Screws Socket Countersunk screws are all A2 grade stainless steel with the recognised standard - DIN 7991. This grade of screw has excellent corrosion resistant properties with added strength over low grade self tapping screws found in most RC model kits where the screws fix in to resin components.