Don't try to unscrew without knowing more, it could be the a hydrostatic pressure relief valve that is used for when you drain your pool to make sure the shell doesn't pop out of the ground....although those aren't usually on a step....what is the diameter and are there anymore of these in the pool?
Disregard my post, I think the others are correct with an umbrella sleeve. Usually railings will have two holes. Bet that umbrella sleeve will come in handy for that AZ sun!
Hello , This thread has been inactive for over 60 days. New postings here are unlikely to be seen or responded to by other members. For better visibility, consider Starting A New Thread.
Welcome to TFP! For a wealth of pool care information please check out our free Pool School. If you are in need of specific help then we encourage you to Register and ask us here in the forum!