matched-hole -1 kit?
Your question must be specific to the matched-hole kit, because in the older kits, the F-833F is just a small doubler that reinforces the rivet attachment through the fiberglass upper seat back to the lower aluminum seat back.
I made much larger doublers that extend farther down the sides of the seatback, carrying more load to reduce the stress and flex at that joint.
In either case, the doubler goes on the outboard side of the assembly.
This may not help you at all -- but for anyone building an older kit, it is a good area to deviate from the plans and reinforce a bit better.