This is a 3/8-24 UNF Nyloc nut, most commonly used on handlebar mounting clamps on Triumph twins from 1969-1970.
The Nyloc nut has a nylon insert at the top of the nut to help prevent accidental loosening of the fastener due to vibration. 3/8-24 TPI UNF nut.
The nut has a locking feature designed to prevent vibration-induced loosening, or in applications where the part pivots and cannot be fully locked down, such as on a rear shock absorber bolt.
Note: Nyloc nuts are not recommended for use in applications where the temperature of the bolt exceeds 250 degrees F.