Yeah, on the XC-LTD, one side was different than the other. The spring on the "big" side didn't plug into a hole in the frame, it plugged into the part you're missing, which I seem to recall was chromed steel. That missing part could be rotated to vary the spring tension on that side, serving as the brake-centering function. When the brake arm was clamped to the frame, that part was immobilized, locking in the spring setting.