There are serrated washes made specifically for brakes, but I don't think the pivot bolt is moving. Friction alone has served to keep these in place for a century, and there's no reason it can't do so today. On simple test is to tweak the light bracket and see how easily it moves. If it does without great force, you'd want a serrated washer behind it, withn the brake depending on friction alone.

But even light brackets don't move unless moved or bumped, so you need to look elsewhere before "fixing" what ain't broke.

The most common cause of single pivot brakes going off center is friction between the spring leg and arm. As the brake opens and closes the spring moves up and down the arm slightly, and it takes very little change in friction to create an unbalance.

Put a drop of thick oil or grease (or Sun Tan oil if that's all you have) where the spring sits on the post on each arm, and work the brake a few times to see where it wants to center. Then loosen the mounting bolt, rotate the brake by hand to center (open), tighten slightly to hold and recheck. If it's right, tighten the rest of the way while holding the brake . (If you have trouble holding the brake from rotating as tightened, use a screwdriver inserted into the spring's coil as a lever to steady the brake from turning as you tighten.

Now recheck by opening and closing a few times, and it should be fine, unless bumping the light backet moves everything (that's where a serrated washer will help).