Large O ring on dizzy? Rubber or other

I just removed and replaced the Electronic Dizzy on my Slant 66 dart. The hold down bar seems to be held (lightly) in place by a large O ring. This O ring is very hard, and has a crack in it. My questions: Is this supposed to be a rubber O ring? Or a hard o Ring (like metal). Then there is also a small narrow slot on the shaft that has a thin rubber O ring, which is rubber. But it is much narrower that the actual slot. Is the wrong size?? BTW this dizzy is a 1970s Chrysler NOS with vacuum pod.