The Vixen Tube Ring Accessory Plate joins Vixen Tube Rings at their top, allowing installation of accessories such as cameras and guide scopes atop the pair of tube rings.  The standard 1/4"-20 threaded screw for camera accessories sits in the center of the plate, so you can use this like a piggyback camera mount on an altazimuth mount.

Alternatively, if you're mounting a camera and using the Vixen GP series or Sphinx series equatorial mounts, you can use the #3796 Weight Shift Camera Bracket to mount the camera alongside the counterweight(s).