The groove down near the gear is actually the one that intersects the oil gallery. On the 265 engines from the mid-1950's, there was a flat spot on the housing instead of a groove. If the distributor was clocked wrong when the engine was assembled, oil would be cut off to the passenger's side hydraulic lifters and the engine would make a terrible racket. 283's got the full circle groove to prevent that from happening.
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!