Looks like hydrostatic relief valves/plugs to me. If you have a high water table, draining your pool can result in the pool shell literally being pushed up out of the ground by the surrounding water pressure. In such cases, hydrostatic relief valves are put into the shell, which allow water pressure to equlaize in the event that the pool needs to be drained for repair, replastering, etc. Sometimes they're an automatic spring loaded check valve type device, and other times they're simply a hole through the shell with a plug that can be removed.