On this parts fiche the Bolt-socket is the most specified component.
A sample from the products for the Footrests include: Pin,12x37, Ball-steel,1/4, Step,fr,rh, Step,rr,rh.
The Zx550a4 Gpz550 1987 United Kingdom Fg Footrests consists of fifteen pieces with the Step,rr,rh being the most expensive.