Every one of the 737-900's would have undergone multiple heavy maintenance checks since being delivered over a decade ago which means the bolts/plug would have been inspected multiple times. If there was a systemic issue in the design where the bolts are becoming loose over time, it would have been identified multiple times over by now. My understanding is the 7M9's that have been found with loose bolts are more recent deliveries that haven't had the heavy maintenance checks which would have required exposing the plug behind the interior paneling, so the bolts are likely still in whatever state they came on delivery.