The only reason there's a groove in the housing is to prevent oil from working its way up due to splash in the crankcase and leaking down the side of the block. Even if a square-sided ring was fitted there, it would interfere with the movement of the housing in response to the vacuum advance.
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!