That seems doubtful since it's not happening on airplugs in other planes. Sure a third party could have failed as well, but Boeing's currently suspect inspection standards are far more likely to fail (if true and with the incentive to cut costs as much as possible) than a 3rd party whose sole reason for survival/earnings is to get paid for doing inspections. I trust greed more than I trust a "gentlemen's agreement" to do the right thing.
