How common were brake boosters?

[dmjlambert]

I am reading on Wikipedia about our trucks and researching disc brakes. "Braking controls included front self-adjusting disc brakes with rear finned drum brakes and optional four-wheel hydraulic Hydra-Boost or Vacuum-Boost power assist."

The footnotes point to factory brochures. And this other source of brochures. The brochure for 1973 says "Brake power assist is standard on all models except Series C10 where it's available." 1971 brochure says standard is "single piston floating caliper front disc brakes (power assist standard on 20 and 30 series)."

I'm wondering if those who have a lot of experience taking apart 71 and up C10 trucks to get disc brake parts can tell was the average truck from the early years (perhaps 71-76) manual disc brakes? Can you give a rough percentage of the ones that had the power assist option (boosters)?