M3 Button Head Range ISO 7380 ~ Socket Button Head A2 Grade M4 Button Head Range ISO 7380 ~ Socket Button Head A2 Grade Socket Button Head Screws Socket button head screws are all A2 grade stainless steel with the recognised standard - ISO 7380. 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.