Rear seat back.
My old RV-3 seat back failed. The seat back would fall through the openning. The fix was simple, re-enforce the sides of the seats and the bulkhead. There's a similar RV-4 CN for the bulkhead, which is incorprated into the plans already. I basically did the same to my RV-4.
To answer your question, the points that takes the loads are the cornders where the top arch intersects the verticals. the side-rails of the seat-back touch there. The top of the seat-rails for a "tab" there. That tab really isn't enough to properly take the load of a 200lb passenger under 4Gs (or 6 for an RV-3). I simply doubled it right there with a small bent piece of 0.032.
.02
-Bruce