Join Date: Feb 2004
Location: Lexington, Kentucky USA
Those are ribbed plastic push knobs, that pop onto the steel finger in your photo. And they sometimes crack because they fit so @#$*& tightly. I know because a had a perfectly good one crack when I tried to pop it on. I wound up filing the raised indentation down to make it an easier fit, and gluing it on with black silicone.
And they're hard to find, which I'm sure you realize by now!
Lexington, KY USA