BOLT 1/4UNC X 5/8

This is a 1/4 UNC by 5/8 inch bolt, part number 14-6102. It's a general-purpose fastener that turns up across a handful of different jobs on the bike rather than being tied to one specific bracket or component. In the catalog it shows up in crankcase assembly, in the ignition coils/horn/rectifier/zenor diode mounting, and again around the fenders and tail lamp, so it's one of those bolts the factory used wherever a 1/4 UNC thread of this length was needed.