Following
@prp mention of screws and nuts, I did more reasearch...
I've see this picture in the past in quite a few occasions...
It's a bit difficult to see but there is some indications from the picture itself...
I believe the chassis plate was fixed with simple tapping screws. We can see the rear sway bar, and there is a screw on the front shock tower that let me think there was a front sway bar. What I don't know about is the link between the rear arm and the chassis plate : toe-in adjustment? Something else?
It would be great if I can find more pictures of this prototype, idealy of better quality...