Nikon thought they were so cool just because they made their BR2 lens reversing ring, so Canon was all like "eat it, Nikon" and made their own reversing ring for FD mount... WITH A HELICOID. HAAA. Mounts to any FD or FL body or bellows, and accepts lenses with a thread diameter of... let me see what it says here... 48mm? Okay. It's in excellent condition, with its original manual.