The distributors with no vacuum advance were used in heavy truck applications where the gas mileage gain with a few more degrees of spark advance at light throttle was not a consideration. 1 1/2 and 2 ton Chevy rigs and heavier capacity GMC's seldom if ever ran at part throttle or less where enough vacuum to operate the advance can was available, so there was no need to have that function on them.

"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!