That's a very cool bike!
Take a look at this diagram of a Mavic brake.
Mavic Technical Manual Update! ~ tears for gears
The main post (part no. 451 015) has two faces or flat sides. Once the brakes have been tightened, use a flat cone wrench (probably 13mm) and center the brakes. The brakes should also have a washer with knurled edges between that post and the brake bridge/fork.