The bolt on the left is correct , the idea is just the when you undo the side with the QR on that piston and the booster come away from the bike leaving just the left piston mounted to the bike .
Seems most likely explanation, esp if bought from someone who removed them from a frame and didn't spot the wee bit screwed into the stud.
Do you have the plates that fit onto the canti-stud behind the units?