The Cavision 6mm Thick Threaded Step-Up Ring -- 77 to 82mm lets you use a 77mm lens with a Cavision clamp-on mattebox that has a 82mm rear diameter.
Both the front and rear of the ring are threaded. The rear thread screws onto the lens you want to use. The front, however, doesn't screw onto the mattebox but rather clamps onto it (and gets secured tight by a pin on the mattebox). The reason the front is threaded though is so that you can also employ the ring in uses other than with matteboxes, such as to connect two lenses together or to attach a filter.
The ring comes at a thickness of 6mm.