As has been alluded to, it looks like the most likely explanation is Boeing neglected to put any bolts in! Of course, there will have to be detailed investigation to confirm if any bolts were fitted, but the pictures do not appear to show any evidence that any bolts were there at all. Other unlikely possibilities could be that the bolts were severely over torqued and fractured after a few pressurization cycles. The evidence may have been blown out the door. It highlights the importance of finding the dummy door plug to see if there are any remaining bolt stems or none at at all. Either way, it is likely the Boeing will have further explaining to do in respect of their procedures, safety and quality control. It is incredibly lucky that the incident happened at relatively low altitude. It is worrying that there have been a series of issues on this type that have, or could have, resulted in fatalities. I avoid these aircraft as a matter of principle. In my opinion, Boeing have got away with knowingly putting peoples lives at risk and there have been insufficient consequences. Passengers should vote with their feet and simply stop flying 737 Max as a protest.