"Zhadman"
It's only a centering device for the brake disc, and it is that thin to save weight...
"zcarnut"
The Z31 pattern is 5x4,5" (5x114,3), and BMW is 5x120mm (5x4,72"), so they are not interchangeable.
This is taking some time, but the cost is almost nothing.
I will be running 8x17" wheels up front with 312x20mm brake discs,
and 9,75x17" wheels at the back with 300x10mm discs