Originally Posted by narvik

What struck me, is that the design of this plug is really meant for it to be very easily opened & removed. Far more so than I had ever expected.

What ultimately holds the whole plug in place, preventing it from becoming separated from the plane in-flight, are only 4 bolts, each secured with a locknut (castle nut) and a cotter pin (split pin).
A locknut (castle nut) is only ever meant to be installed with the addition of the cotter pin (split pin). Whether the cotter pins (split pins) were installed or not (and of the correct material and size) is what will be one of the main questions that will need answering, as that is the most likely failure here, IMO.

Not particularly impressed with the design myself. Still trying to understand the frequency of how often United (or Boeing) has to remove this plug throughout the lifespan of the aircraft. I'd have thought only a few times at the beginning and potentially at the end of its United tenure?

A plug in a pressurized cabin, that is required from not moving outwards, can easily be achieved by making it larger on the inside than the opening itself. This whole idea that the plug is so easily removed outwards and effectively a door that's held in place with 4 bolts is astonishing (to me), despite it apparently being done so for hundreds (thousands?) of frames.