This Cavision 6mm Thick Threaded Step-Up Ring lets you use a 58mm lens with a Cavision clamp-on matte box that has a 58mm rear diameter, or with a 58mm filter.
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 matte box but rather clamps onto it (and gets secured tight by a pin on the matte box). The reason the front is threaded though is so that you can also employ the ring in uses other than with matte boxes, such as to connect two lenses together or to attach a filter.