This replaces the old RF box to provide composite video output. It's based on the Project Speccy design. Before you purchase by absolutely certain you know what you need and are doing as this requires a small amount of soldering.
The existing RF box has 4 connections. As you are looking at it from the top, to the lefthand side the first connection around the middle is the video in. The 2nd connection on the lefthand side near the bottom left is the 5v line. Underneath the RF box, there are 2 ground pins. Desolder all these and remove the RF box. Now solder this mod in its place using the same connections.