Hi Mark
Firstly the two parts you show are from different headlight systems. The ring is from a Carello light fixing and the plastic clip is from a Siem one. Both are from the individual light setup as opposed to the combined one found on some of the cars.
So on to how to establish which is which as often the main bowls may have been swapped out on the Carello lights, though not on the Siem as the fixings are pretty unique I think.
The Carello lights consist of a backplate attached to the main body of the car, and then the light bowls are attached to a metal outer bowl which then attaches to the backplate and has a tensioning spring and a couple of adjuster bolts. The Siem system is much simpler and has just the backplate and the light bowl uses the 3 of the clips above to locate one in the backplate and two adjuster bolts.
The outer ring for the light bowl on the Siem lights simply has bent tabs to hold it in place on the light bowl, the Carello one uses the tabs with holes at 90 degrees to the rim to screw to the metal backplate. On my Spider the outer rings are from a different car/supplier and had been modified to fit.
Hope that makes sense.....
Peter
