Hi guys, I had the rear cap stuck to the lens. Then, I removed two screws on the silver ring---biggest mistake. After the screw is removed, a metal stopper fall off. So I unscrewed three screws at the rear to take off the next layer to get the stopper out. Now I ended up with a lens that I can't put together. 1) The location and the direction of the metal stopper is uncertain. 2) The relative location of the aperture ring stopper is uncertain How should I put them back? Helps are really appreciated!