This sight hole cover threads onto the back end of the Right Ascension axis of the following mounts:
all 400 mounts
all 1200 mounts, except the original black, metric mount
all 900 mounts shipped prior to October 2005
One of these aluminum covers is included with all new mount purchases, but now they are available separately for anyone who needs one.
The rear of the 900GTO R.A. axis was redesigned in October 2005 to be slightly larger than the 1200s, 400s and earlier versions of 900s. This change resulted in the need for a larger diameter cover.
If you are uncertain when your 900GTO shipped, please use the photos (right) as a reference for where to measure on your mount to determine the correct cover size. We used a 1200GTO since it was readily available; however, the measurement points are identical on older 900 mounts.