Buddy seat measurements (MG# 13460545)
Moto Guzzi V700, V7 Special, Ambassador, 850 GT, 850 GT California, Eldorado, and 850 California Police models
Updated:
MG Cycle sells a reproduction buddy seat for the 850 GT California and other models.

Photo courtesy of eBay auction.

Photo courtesy of eBay auction.

Photo courtesy of eBay auction.

Photo courtesy of eBay auction.

Photo courtesy of eBay auction.
Thanks to John Appleby for posting this information on the old Yahoo! Loopframe_Guzzi news group (which has now moved to Groups.io). In John's own words:
Shape of the seat pan is going to be tough to create a pan with but here are my rough measurements of the pan:
- Front to back: 21 1⁄2 inch
- Side to side at the rear frame mount: 13 1⁄2 inch
- Side to side at the front frame mount: 7 1⁄2 inch
The pan has a lip around the sides (not in front) that is about 1 inch high.
The back of the pan bends up approximately 3 inch starting at the rear seat mounts. There is a 4 inch square piece of rubber centered on the pan starting about 1 1⁄4 inch from the back of the pan to protect the rear fender.
The front frame mount is a U shaped affair that is 2 1⁄8 inch wide and 1 1⁄4 inch high with a 3⁄8 inch slot that goes down 5⁄8 inch for the tank mount bolt. The front mount is centered on the pan 2 1⁄4 inch from the front of the pan.
The rear mount is a 3 1⁄4 inch piece of steel bent up 90° at 1 inch then bent 90° forward at 1 1⁄4 inch with a bolt receiving hole 1⁄2 inch from the front of the piece and a nut welded to the bottom for the bolt to screw into. The rear mounts are welded about 13 1⁄2 inch back from the front of the pan, this can be better measured by the mount holes on your bike.
Both front and rear mounts are steel stock 1 inch wide by (approximately) 1⁄8 inch thick.
The mount holes for the chrome bar would be best measured from the piece but the bolts on mine are welded 14 1⁄8 inch back and 3 1⁄2 inch from the side of the seat pan. The rear most mounts for the chrome rail is 19 3⁄8 inch back from the front of the pan and 1 3⁄8 inch from the back of the pan. The measurements are approximately on center and the bolt is about 3⁄8 inch (with a 3⁄4 inch nut).