This quadrant specifies the fastener's basic part number (often or MS standards) and the material it is made from.
The NAS523 code is a standardized practice that includes all types of fasteners that undergo permanent deformation during installation. Removing these fasteners typically requires their destruction, making precise initial installation essential for structural integrity. nas523 pdf
Often identifies an MS20470AD rivet (2117 aluminum alloy). This quadrant specifies the fastener's basic part number