[markeb01]

Don’t know if it’s the same thing, but back in the late 60’s or early 70’s, I was shown something similar that a guy was adding to his 66 Mustang fastback. It was a metal box several feet long and very heavy. According to the owner it was a new anti-sway device containing a heavy steel weight retained by springs on each end and suspended in hydraulic fluid.

The theory was inertia would tend to retain it in constant motion while the rear of the car was flinging back and forth in slalom maneuvers. I never heard any follow up whether it worked or not, and I never saw another one.

I believe the principle was similar to the old Gyroscopic wheel stabilizers. Those were clear plastic tubes like a wheel sized hula hoop filled with ball bearings and hydraulic fluid. They clipped on the inside of the wheel rims and were claimed to attain self balance at any speed as the balls would fling around until they achieved neutral balance. They were a popular gimmick for a few years, but I have no idea if think they actually provided any benefit.