Originally Posted by G550Ted
It appears to balance on the pads at the top of the support arms. The small, ball tipped "feelers" at the ends of the scaled bars would be used to determine the distance from the pivot (CG) to a reference point on the model such as the leading edge of the wing. I may be wrong, but don't think one would place a 40# model on those tiny balls - ouch!
Teddy, have a look at this video (about 1.25 min mark onwards) and you will see this cg machine in use. It appears that the plane is placed on the small ball tipped "feelers"