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).
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"