Study the photos in this link:
https://www.beverlyhillscarclub.com/...er-c-17587.htm
You will notice a footman loop just ahead of the rear seat on the tunnel hump and two flush fittings on the rear corners. It appears that this may be the three positions that hold it. It had to be easily removable to put the top up and down. I don't know with any certainty how it works but there is that evidence to look at. There may be a parts break down on some of the Ford Thunderbird parts catalogs but I sold my 64 T-bird and all my books for it went along in the deal.