My Hegner Multimax-2 is made differently. There is spring tension on the tension arm, but at the pivot point.
The black bracket on your looks like it might have been added to solve a bearing problem farther back on the arm? To the left of the tube, it also looks like the arm might have been welded. Can't tell from the pix. Check the arm to see how much lateral "slop" there is.
Assuming theres a small space between the ends of the tube and the bracket, you'd probably want a ball bearing on both sides that will just fit inside the tube, but not fall through the spring. An auto parts place might have them, if you don't find them on the floor.