I believe this is a .8 zoom ring for Angenieux 15-40/28-75/45-20. It was in a box with other Angenieux Zoom Lens parts but I am not 100% sure that I am correct. If you're interested in this, or know for sure what it may be, let me know. 

I can put some calipers to it and provide a more exact diameter (when I find my misplaced calipers)