Installing rear seat shoulder harness P337 ?

[AV8ing]

We just sold the Riley Sky Rocket P337 that I look after and the "new" owner asked to install shoulder harness for the rear seats ( 3 & 4th).


I checked the IPC and indeed there are PNs for them, but when I pulled the headliner to check for provisions, I found just a small 3/16s rivnut . A quick check of the pilot & co-pilot shoulder harness provisions shows 4/16s nut-plates in a beefed up bulkhead member.

Clearly this rivnut is not structurally sound to handle seat belt loads.

Readers ?