A sliding goose neck shouldn’t have thread at all. The wing bolt goes through it and screws in to a sort of nut that has a shape to fit the inside of the sail groove. When tightened it jams the sides of the sail groove. Because it is shaped to match the groove it holds well because of the large friction area, and it does so without damaging the mast like a straight bolt would do.
If you lost the “nut” you can replace it by some half round brass or stainless steel strip (don’t use aluminium here), similar to the keel strips on your boat. If it is about an inch long and more or less follows the curvature of the inside of the sail groove it is fine. Drill and cut thread in it to match your wing nut. Alternatively you could grind a standard SS-nut to match the inside of the groove (more or less) but that would not have enough surface area to stay in place when loaded by the sail in a fresh breeze. But it is probably good enough to potter about on a lazy Sunday afternoon.
I believe someone mentioned the use of round bar to me once?
Just use what you can find in your pile o’ junk that matches the sail groove, or rather, creates the largest friction area inside the groove.