I've never used the centering screws on any double pivot sidepull brakes, including a set of Dura Ace brakes just like the ones you picture. I adjust the rim clearance by setting the cable length and fine tuning it with the cable adjuster and I center the pads just by twisting the entire caliper by hand then tighten the brake nut fully. If the screws in my brakes fell out or stripped, I'd never miss them.