Some years/models have screws, some pop rivets. You probably have another on the underside of the left chainstay, or perhaps an open hole. I don't know why they had them but assume it had something to do with the manufacturing process. They may have filled the stays with Argon when welding to prevent weld contamination in case the weld penetrated the base material completely. If filling with Argon they would have needed a hole at each end of the stay. The front of the stays are vented inside the bottom bracket shell.