Large aluminum pop type rivets.
If I remember correctly, you have a couple of large aluminum pop rivets holding that block into position. You should be able to buy identical pop rivets of the same size from Home Depot, or most other hardware stores in their specialty fastener section. It may require removing the screw-on tip from the pop rivet gun to accommodate that larger shank of the large pop rivet. Be careful not to elongate the hole in the mast when drilling the old rivet out or the new rivet will not set properly.
