Ebb, I didn't invent this gate design. I roughly copied this from another S-Boat's mast. You are correct, the fixed thumb wheel and the pan head sandwich the sail track between them, allowing the gate to open and shut. The pan head machine screw is threaded into the lower bronze plate which is screwed to the mast. A shallow hole in the wood under the plate accepts a length of the machine screw when the gate is shut.
Incidentally Nat Herreshoff invented the sail track used on the S Boat and the Ariel. I've always found that strip of bronze going up the Ariel's mast pretty classy.
