missing small oem roofrack parts

My new 89' rt4wd has the factory rack (yay!). The PO had took off the top rails. The guide rails are still on the car. I found all the knobs to tighten the top pieces into place. What I think I'm missing are what the knobs screw into. There must be some small threaded pieces that slide in the rails for the knobs to secure to.
What do they look like?
What do they look like?
Comments
this is actually the rack that's on dizzle's sled. There's a square "nut" piece, then a plastic slider under that in the track:
kinda crummy pics, I know... these were ones I took as reassembly instructions when I shipped it. I *THINK* that Aerostar or similar parts would work, they look very similar. The square nut thingies that come with a Yakima track mount will work, too.
thank you for the pics!
I noticed on the rack that the two larger cross pieces both have threaded pieces in them. Did honda offer options to add to the top of the rack? If so what...... and I want it!