Factory. The square plate is usually a piece of fibreglass and held on by a self tapper at each corner to cover the round hole. It’s a handy access panel in the rare instance that you do need to drain the tank. It’s easier to keep it than it is to glass up the hole!
On the basis that my tank doesn't have a drain plug I glassed up that hole to close up at least one of the many opportunities for moisture to get into the car. If I need to drain the tank then I can always use the fuel pump!
