Stainless steel; required equipment on the Skerry, Northeaster Dory, PocketShip, John's Sharpie, and Mbuli Proa booms.  Mounting plate measures 1-1/2" x 3";  fasten to mast with #8 panhead stainless steel screws. Half-inch diameter tang/rod engages boom.  The universal joint has enough play that you can sail 10 or 15 degrees "by the lee" without damaging your boom.