I'm on the road for a few days. I'll post some pics when I get home showing the original coupler, and if I can find them, the updated rigid coupler.
The original uses 4 ea 'rubber'~2" dia discs (donuts) on ~6" radius. They seem to start getting tired at around 500 hrs on the 13B/Renesis.
The rigid switched from an auto trans flex plate to an aluminum racing flywheel (+~8 lbs), with roughly the same size input shaft.
Same 6 pinion drive on 20B, but the P-port 20B guy had to beef up a lot of the hardware 'around' the planetary gearset. Never had any issues with the gearset itself, though.
My layman's understanding is that (on the 13B rigid) lash handles keeping resonance below idle; iirc, it will start to 'rattle' below ~1200 engine rpm. Of course, there's minimal impulse strength at that rpm/prop load. Remember, torque never goes negative with 2 or more rotors, so the gears never see reversed torque at normal operating rpms.
Edit: RE, quill shaft action: IIRC, at one point during discussion on the Flyrotary list, Tracy ok'd modifying the original box to using the original input shaft direct coupled to our choice of an aluminum or lightweight steel racing flywheel. So quill shaft style decoupling seems unlikely.