Boeing quality control could be hit and miss or its design engineering is lacking

Further investigation points to that Spirit the maker of the 737 airframes do ship it with this blank in place but only temporarily fastened. Boeing then take the blank out to load the interior and then reinstall it. 

The blank is like a door with a larger window and no manual opening mechnism and is being held in place by a frame rail with lugs but instead of movable pins and a handle like a normal door it is held in place with bolts and castelated nuts that should have split pins in them. As long as they are in place the doorblank can not be moved upwards (some say downwards and some later pictures of a locking bolt seems to support this) to a position where it can be opened / taken out. Again, regular inspection of these bolts have been made inpossible by having a complete panel on the inside.

Questions remains over if split pins where installed. Bolted on parts can have the nuts unwinding themselves due to vibration. And the exact torque required to prevent this can be difficult to calculate. Other applications might use locking washers, threadlocking compounds and/or even dobbel nuts for a longer grip on the bolt. And even then regular inspections might be required for a long time to check if there is any movement. 

Now other airlines have alo reported that on inspection bolts where found to be loose.

Here where a bolt probably was not required to be regularly removed some sort of deformation of the thread outside the nut could also have been applied since the most important was that it didn't fall out and not that it was properly torqued. As long as at least 1 of the 4 bolts was still in place the doorblank could not slide up and fall out. This would make inspection at checks for exmple corrosion around the hole more difficult but not impossible.

The bolts could also have been overtorqued and then simply broken after a while. There is also questions around why the plane retained the spring mechanism that when opening lifts a normal door out of its lugs alternatively lifts the rollers into them. Convenience, laziness or that was how the door was approved so that is what is also in the plug even though its not needed and could present a risk.

There where however leads to that something where a miss due to several pressurisation warnings on flights prior. Questions is also coming up on why the 2 seats closest to the door where empty. Could they have been blocked off / deprioritized due to noise coming from the door. Even the smallest gap would lead to excessive engine noise being heard. Even though its now rumoured the passenger destined for those seats very luckily missed their connection.

On a positive note, Calhoun the CEO of Beoing did come out and say that this should be a focus point for the whole Boeing organisation. Which is understandabel because body panels falling off mid air on such a controversial plne is not a good look. And a lot of luck was involved when the accident did happen in that it happened on a short flight due to the plane being shifted from its normal hawaian route.