Harley-Davidson OEM Part #7720 is a chrome Acorn Nut / Locknut with a 5/16"-24 UNF thread pitch. 
Part Details
  • Description: Nut, Acorn / Chrome
  • Thread Size: 5/16"-24 UNF (Fine Thread)
  • Finish: Chrome-plated steel
  • Common Applications: Used extensively across various model families—including Softail, Touring, Dyna, FXR, and Sportster. It most notably serves as the mounting fastener for handlebar-mounted mirrors (from 1982 to present models