This SimRing is custom-designed to fit on the nFD/FD 15mm f/2.8 Fisheye Lens.  It slides over the front of the lens and is held securely by 3 supplied set screws. 
 
NOTE:  
This ring will cause vignetting when shooting Full Frame.  To avoid vignetting, 1.4 - 1.6x (S35) cropped sensors are recommended.
 
Specs:
95mm Outer Diameter w/86mm filter threads
Air-craft grade aluminum and anodized in black
3 set screws and 1 tool is supplied
 
Don't forget to purchase a custom 95mm Front Cap!