The stem on those don't look long enough to me. But it is hard to tell. I have always had enough spare parts to replace missing knobs, so I haven't researched this issue much.
I have the same doubts that Bruce has. I guess maybe the small round things are intended to pop in where a stem is needed. I do think they look quite a bit like the ones that came with the very early Genesis that had wide slats. Unlike all the rest of the classic Genesis they had black knobs with the same white marker.
I used Weber OEM black knobs and for the side burner one new Weber gray knob of the same design spray painted black. These were the last knobs that came with the 2005/6 Silvers. They are larger and a bit more substantial, but they have the same stem as the original knobs. I think they were trying to keep up with the competition that was starting to feature much larger control knobs.