That's where my hydraulic swim lift controller is. The screw above it has a strap on mine that snaps to the door to hold it open as you stated.The weatherman lied to me today, said 65 and cloudy and it is 76 and sunny. Ughh, so I am faux boating sitting on the boat at my dock.
That said, starting to get down to the real little details to fix.
What the heck used to be here. Assuming it was to secure the door to the swim platform open??