I believe it's the signal pin that communicates the maximum aperture of the interchangeable lens component to the camera's meter. The shoulders of the 35mm, 95mm and 125mm components (behind the lens mount threads) push the pin further into the lens than the 50mm (signaling f/3.5 vs f1.8 maximum apertures). Canon's FD lenses have maximum aperture signal pins that perform a similar function with FD bodies.