On this parts fiche the Nut,rr View Mirror is the most specified component.
The most reasonably priced component used on the Gsxr750 1988 (j) Usa (e03) Handlebar (model L) is the Pin.
A sample from the products for the Handlebar (model L) include: Mirror Assembly,rear View, Cable Assy,throttle, Mirror Assembly,rear View, Screw(6x127).