At the bottom of a box I found two plates. The box is related to the chassis of the car but I had no clue whatsoever where to put them. I had two pictures of them, one before cleaning and one after cleaning.
The first put them in a context of the forward stabilizer bar and the second in the context of the rear axle.
Scanning the parts catalog did not help. Fortunately there is always the SDC forum. It took a while but the winning tip was from Brad Bez. They are reinforcement plates that go between the U brackets of the forward stabilizer bars and the frame. The hole in the middle takes the forward bolt of the body and thus forms a reinforcement.
I also installed the front springs. This is an interesting procedure which normally uses the weight of the car to balance the jack. Since I still miss this weight I had to use a pair of threaded rods and nuts to pull the lower A-frame up against the spring force.
Just make sure the springs stay where they are. The forces are great so I think they would cause considerable damage when they start flying around. All went well so in the end the lower hinge was bolted to the frame.