The Movcam 114:100mm Step-Down Ring for 114mm Threaded MatteBox is a threaded ring for threaded 114mm matteboxes. It screws onto the rear adapter of a threaded 114mm mattebox and lets you mount a 100mm lens into that mattebox.
Movcam makes matteboxes in four different sizes -- for 144mm, 130mm, 114mm, and 104mm lenses. All but the 114mm are clamp-on. The 114mm is threaded.