From AP's website:
Model 24" Ribbed Plate (1200RP)- 1200
24" Ribbed Mounting Plate for 1200 Mount for 180EDT/EDF, 206EDF and other large instruments.
"For larger instruments, the ribbed structure of this plate provides the maximum support. Again, we start with thick aluminum plate and carve a strong rib structure. The final result is 1.5" thick, in the center, 24" long and 7.6" at its widest point. The width of the plate tapers to 5.5" at each end. A pair of keyhole slots that measure 3.2" between centers are provided at each end. The distance between these pairs of holes is 23". Due to the ribbed structure, you may not be able to drill additional holes to suit your mounting rings. The plate only weighs an amazing 9.5 lbs."