Something brewing behind the scenes?, page-43

Yes they obtained approval for the 737 Centre wing box application - my understanding around this was that the approval was more centered around defining an approval pathway for other applications that were more economically appealing to airlines and in hard to access areas.

The 737 CWB inspection from memory was not as frequent as say the APB or WiFI inspection and is needed close the end of the planes lifecycle which probably makes a cost-benefit to airlines next to nil.

None the less it has helped define a way to approve the technology for the next lot of applications where there is value to airlines.

https://www.osti.gov/servlets/purl/1660800
https://www.osti.gov/biblio/1660800-application-certification-comparative-vacuum-monitoring-sensors-structural-health-monitoring-wing-box-fittings

Further reading in the above link