Monster headlight buckets:
Plastic bucket, the lens/reflector assembly attaches to the chrome ring with 3 radial screws.
Metal bucket, lens/reflector assembly is held into the chrome ring with spring clips.
Pick the adapter that will fit your stock bucket pretty much. Each chrome ring was a slightly different size too so Motodemic had to make two different adapters. I have one of the first prototypes and not sure if the design has changed much since then.