Dave, so far as I know, you can't install a ground glass directly into the Horseman rotary back - it has to be installed in the appropriate frame first. The instructions linked by Bob show an entire 6x9 focusing back complete with gg frame and hinged fold-out hood assembly installed in the rotary back. This can be purchased as a separate accessory - about $400 now! - but more commonly the user probably just takes the back off his VH or VH-R and swaps it to the rotary back. I've also seen a much simpler device that's just a metal frame with a ground glass in it, which I presume is held in by the Graflok clips rather than by the hook-and-pin arrangement that holds the assembly from the camera back. Perhaps this was targeted at users of the 4x5 version of the rotary back, who didn't already have a VH or VH-R from which to borrow a 6x9 focusing back.