[svHyLyte]

... It appears from the photo that the tightening of a knob on this bolt would hold the gooseneck in place, while loosening could allow a dwonhaul (sic) adjustment. ...

Correct...almost. It allows the adjustment of the positioning of the goose neck. There is no fitting allowing for a down-haul although one could be added. A Cunningham would be more effective for adjusting luff tension however.

 

[svHyLyte]

The tab is one piece with the metal band.
And yes, the gooseneck sides up and down freely.

I thought the knob might hold it in place (and I can't imagine what else it could be), but putting a knob on and tightening it does nothing at all. That is what makes me think a piece is missing. Maybe a U shaped piece that fits under the knob, with the two arms pressing against the mast?

I believe that the threaded rod projecting from the tab within the slot once had a knob affixed to the end with which one screwed the rod through the tab and into the slot until the inner end-face of the pin came into contact with the inner face of the slot, locking the goose neck in place. If so, there may be slight indentations on the inside facing of the slot that could be observed in good light. The rubber grommets would have prevented the stop pin from being over tightened and distorting the slot.

FWIW...