It looks like the ear on the left side of part 15, which I've circled in blue below, slides into the receiving slot that I've circled in red on the big plastic piece. Then the screw goes through the big plastic piece, the other hole in part 15, and into the speed nut, #16. I changed one of those a few years ago and seem to recall it going like that. Take a look and see if that works.
Threaded Mode
