This came from a commenter at the aviation subreddit.
> The much older 737-900 shares the same door plug assy as the 737-9MAX. No other models do.
> Spirit says they "semi-rig" the door plug before shipping. To me this means it's not fully installed. The reason is because the door plug assy is removed upon receipt to gain an additional access point for final assy work.
> However, I bet that's only true for the 737-900 and that for the 737-9MAX the final assy work is completed without ever removing the door plug.
> The Spirit planner copied over the semi-rig task from the 737-900 to the 737-9MAX. The Boeing planner didn't add a final rig task because the door plug was never removed and they assumed that meant fully installed.
This also would explain something that many find hard to understand - why only the 737-9MAX is grounded pending inspections, not the 737-900 which has the same plug door. Every 737-900 had the door opened and then sealed by Boeing engineers.