Part number SONOR 75823500. Two photos of a partially intact one pulled out of a Designer H-bracket.
They cannot be installed by hand, but need a miniature press. I mounted one using a long thin bolt, two washers, and a nut, with a brass shim wide enough to allow the rubber head to squeeze up through the hole. Even then, with the rubber fully compressed and using a lot of lubricant, I only managed to get a portion of each rubber head through the H bracket, one at a time. The rest of each head had to be levered along under the lip, being careful not to push the thing back through its hole.
It's tricky. To make an effective press, the bolt has to go through the hole in the rubber. Thus, if the bolt is too thick, the rubber will not be able to compress enough to squeeze through the bracket. I'd like to see Sonor's tool for the job.