The base code '20426AD4-3.5' specifies a solid, 100° countersunk precision head rivet ('20426') made from 2117-T4 Aluminum Alloy ('AD'). The '4' indicates a shank diameter of 4/32 inch (1/8 inch), and '-3.5' indicates a fastener length of 3.5/16 inch (7/32 inch). The trailing 'A' in the part number is not explicitly defined in the provided sources as a standard finish or variant code for this specific part number format. While some related part numbers show a trailing 'A' (e.g., MS20426AD4-30A), its meaning for 'MS20426AD4-3.5A' is not stated. MS stands for Military Standard, identifying the part as a military aerospace hardware standard. Governed by the MS standard series, administered by Military Standards.