Not necessarily missing pieces. It is just designed for a different kind of bike.

Lots of more utilitarian Swiss and German bikes have a plate across the seat stays where the brake bridge could go. The rack mounts up to that. So that rack is not necessarily missing parts, your frame might just not have the plate that the rack "expects" is there.

But in the absence of the plate, there are bits as shown that sandwich the seat stays. careful with these. they can crimp the seat stay. Not through direct pressure but through leverage. When the stays on the rack flex or give, then the clamp exerts a lot of leverage on the seat stays.