Alain,
To amplify what Ed said, here are two photos:
and
The first photo shows a single nut, looking from the front of the car toward the rear. The second shows both nuts.
As you can see there is a tube reinforcement going from the front to the rear, and the washer and nut are all the way toward the rear.
Looking at your photo, it seems you are looking at the rear of the crossmember. If so, probably your differential will mount the same way as mine.
A LITTLE CAUTION: My car is an Everett-Morrison, NOT a Contemporary. It does appear the two cars have the same mounting arrangement.
I hope this helps!
Tom