The way the vacuum advance is mounted onto the distributor housing, a gap won't be noticeable until the pressure gauge fails to get a reading. Particularly on the babbit-pounders, low oil pressure is expected, so NO oil pressure sometimes fails to get noticed until it's too late.

"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!