Drgondog, again your superior knowledge has me at a distinct disadvantage. After all, I was just a dumb pilot and all the mechanics would start my day by telling me: "I told Orville and I told Wilbur and I'm telling you; it'll never get off the ground." Well, they were wrong. Getting it off the ground wasn't the hard part: keeping it off the ground was.
Anyway, I didn't realize the G model's rotor system was from the J model. It must have been a retro-fit because I'm pretty sure the models I flew had rotor systems originally from the C model Huey which I also flew (much harder to fly than the Cobra due to weight problems -- the aircraft's, not mine

.
As you say, though, we're both trying to remember 40 years ago (I wish you hadn't reminded me of how much time has gone by

).
Chuck I know the Cobra G rotor was both thicker in chord and greater span than the C. Whether it was a J rotor - I honestly cannot remeber for sure.
That J model must have been some bird with those twin engines, etc. We thought the G model was pretty hot stuff and, compared to the C model it replaced it really was. A loaded C model was almost impossible to autorotate -- it would just fall through the flare and break backs.
Auto rotate is one of those interesting 'concepts' which occasionally works in real life. Every time I see a movie in which a helo loses a tail rotor I cringe
I flew H models and I flew Rangers but, I don't remember the vibration system you mentioned. Here again, it has been 40 years. I do know I suffered high frequency hearing loss in my right ear as a result of that darn turbine whine, though.