The plug was of course installed when the plane was delivered, otherwise there would have been a door-sized hole in the side of the plane when it was delivered.

The bolts of the plug were probably originally installed, but some rework was required in the area during final assembly, which required the plug to be removed and put back, and during that process the bolts were not reinstalled. It’s (lack of) documentation of that removal-and-reinstallation that are the subject of the article we are discussing here.

What I have read varies. Some say the door plug was installed and bolted. Some say the door plug was installed but not bolted. Some say the door plug wasn't installed - which I suspect from a lot of prior experience is a linguistic gap between engineers and reporters on installed but not bolted.

I remember reading but can't find at the moment something suggesting that inspecting the door plugs and the bolts was a required Boeing assembly line 'job' (think 'ticket'). But none of that matters if someone, as has been reported, later uninstalled the bolts to fix something else, without creating a record of doing so, because they misunderstood the specific procedure they were undertaking.