Many NAS standards have been transferred to SAE for maintenance. SAE’s database ("SAE Standards Works") may host NAS523 as a historical document. SAE members can often download PDFs for free or at reduced cost.
A typical NAS523 rivet part number looks like:
NAS523-X-Y
NAS523 rivets are typically 2017-T4 aluminum alloy (common for general aerospace use), though other materials exist with suffix letters (e.g., NAS523-6-8C for corrosion-resistant steel). The standard finish is plain or chem-film coated depending on procurement spec.