This black anodized mounting saddle plate is not specifically listed in AP's web site and so I can only assume that it is either an older discontinued plate or a custom machined plate. It has a circular pattern for AP900 mounting holes and was previously used to mount my Ceravolo HD216 (sold to me along with the scope back in 2000). It may have been designed and fabricated by Peter Ceravolo as part of the original deal to the original HD216 owner but I honestly can't say for sure.
Here are the plate dimensions:
Plate Long Axis: 15 7/8"
Plate Center width: 6 1/4"
Plate thickness: 1/2"
Hole spacing (long direction): 15 1/4"
Hole spacing (short direction): 3 1/2"