The extra lip you speak of is on the early 9" with the weight blob around the edge, not sure about the flat 10" er's I had one with the lip broken off in many spots, some of the now deceased members advised not to use it. I think I have a pic...
Well, I could not find it. Its very thin, easy to break if you don't get the gear warm enough for maximum expansion, and try to hammer the ring off or on.
Kube is correct, its not what hold the gear in place, perhaps to aid in locating it? I dunno, but not used on most other flywheels so Henry must have quit too.
Can't remember if I used mine or not. There was a thread of mine with pics 5 years ago or so.