Part number for rubber bumper on roof rack crossbar?

The roof rack has been randomly rattling. I found I could wiggle the front bar by hand.

I took the bars off and found that this rubber piece was missing. The picture shows the end on one bar with the rubber bumper (right) and the other bar with an empty hole where the part should be (left).

I feel like I'm probably SOL on this, but hope I am wrong.

I am pretty certain this is the OEM roof rack.

Does anybody have a part number for this?

 

For the sake of closure...
I found the part still stuck in the rail on the truck. Reinstalled with the crossbars parallel to the rails.
No more rattle.