[cameldrv]

I've heard Spirit has had a lot of problems, but likewise Boeing itself is not immune to this. A few years back the Boeing South Carolina plant was having a very consistent problem with foreign objects (wrenches, parts, etc) being left behind in assembled planes, to rattle around and damage who knows what.

My understanding as well is that while Spirit makes the fuselage of the 737MAX, final assembly is at Boeing in Renton Washington, and the plug door would have been removed for final assembly and then reinstalled in Renton, so Boeing itself would be responsible, assuming, as appears likely, that the plane left the factory without the door bolts installed.