I got a few more pictures of the mast from a craigslist seller, and based on the rigging instructions there might be a line halyard with a wire leader at the head end, with a little ball or clip that could be pulled over the sheave and then set in the little notched tab at just under the sheave by pulling the halyard aft.?
The CL seller uses a regular piece of line with a bridle clip on the end, and reccomends getting line that is pre stretched, high load (like dyneema) because the downhaul exerts a lot of force on the halyard.
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See attached photo of mast head.
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Signal Charlie