My Spider came to me a year ago in very good condition.
Sorry, but I can't do a picture, but the back of the bottom has chrome(?) metal tabs about 3/4 inch wide, with about an inch long part facing forward. The tab is folded in half such that it clamps to the seat bottom, with both halves facing forward. The top side receives the adjusting bolt.
The adjusting bolt screws into a threaded hole in the bottom edge of the back. It is obscured by vinyl so I haven't seen it directly. A locking nut is tightened to the seat back part, with the head resting on the bottom tab.
Simpler than my description, and works well.
Hope this helps.