If you go to
www.Defender.com and then type in mast cleat, you will find these. They are common. When you go to remove this, the screws will be frozen into the mast, so spray and soak them with PB Blaster from Ace, or Home Depot or any hardware store. Then after soaking for 10-15 minutes put the largest screwdriver into these screws, put pressure to loosen and at the same time tap the end of the screwdriver with a hammer to send vibration into the screw. This vibration and the PB Blaster will break the screw loose. Take your time and this may come right out or it may take you 30 minutes....
search.defender.com/?expression=mast%20cleat&s=1
Hayden