Standard or part number (e.g., "BJ" for a specific rivet type). Diameter & Head

: "Icebox" rivets (2024-T alloy) requiring heat treatment and refrigeration before installation; identified by two raised dashes. Installation & Hole Preparation (SW Quadrant)

The system breaks down technical details into four quadrants (NW, NE, SW, and SE), though these compass designations are typically implied rather than labeled on the drawing: Northwest (NW - Upper Left):

Understanding NAS523 Rivet Code PDF: A Comprehensive Guide

The next letters identify the material the rivet is made from. This determines strength and corrosion resistance.