My '85 GS buzzes at 6800 and up.

 

The rpm at which it goes off is adjustable.

Also, that is the grey box missing on my stock 80 cluster in my car. It was never there or soldered in ever.

 

Been doing a little reading and found this: https://www.rx7club.com/1st-generati...5/#post8623341 which claims that "it is true that only the '79, '84, '85 have it". It being the overrev buzzer.

I'm back to thinking my gray box is the 7V DC regulator for the resistance type gauges (temp and fuel) for 2 reasons. It looks like an old school voltage regulator (if I have the cluster out anytime soon I'm gonna measure for 7 volts OR make the damn thing buzz to be sure) and there is no adjustment circuitry in my cluster for calibrating an RPM threshold.

Okay 3 reasons: I had an 83 GS back in the actual 80's and I never heard an overrev buzzer from that one either.

Edit:
Deep dive on old school voltage "stabilizers"
https://www.allpar.com/history/mopar/electrical2.html
http://www.britishv8.org/Articles/Voltage-Regulator.htm

 

Looking on the thread you linked, it might actually be possible to mod in a buzzer. According to this post: https://www.rx7club.com/1st-generation-specific-1979-1985-18/overrev-buzzer-792565/#post8624485 ]http:// https://www.rx7club.com/1st-generati...5/#post8624485
The guy acts like its as simple as soldering a couple things on and you're good to go.

Edit: Not sure where you would solder in the leads for the rpm switch or the buzzer, but should be quite easy to do once the points are known. I would really like to do this on my 83 GS.

I have no idea what the grey box is, lol. It could be a buzzer, which if it is, might be possible to hook in with the stuff you have to add. If its not a buzzer or not usable for this, can always find a different buzzer.