The vacuum advance on some of the GM distributors has a small flat spring with a ball bearing trapped under it to provide a rolling ground for the moving housing. Others just rely on the contact of the distributor housing in the block for a ground.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!