This comes from a set of Weinmann Type 370 calliper brakes. The two nuts you see are normally firmly located inside the plastic hood, not as they are shown.
I think they are intended to function as some sort of self-locking/clutch system, so that the nuts don't work their way off, while still not preventing the callipers from moving.
In this particular case, I am not satisfied that the nuts are securing the callipers on the bolt as tightly as they should be. As I rotate the plastic hood, the nuts move down the bolt, but then further rotation doesn't tighten mechanism; the hood just rotates over the nuts instead of rotating them.
How is this actually supposed to work?
In the late '70s or early 1980s the domed centrebolt on the standard side-pull calipers was replaced by a plastic cover over two plain steel locknuts. These brakes were centred using an inverted allen key tool.
but that doesn't shed any light on the how the two-locknuts-in-a-plastic-hood arrangement is supposed to work.