Seems to me to be very, very odd that out of the bolts that are visible, only one has a nut. Others appear to have a washer in place and one appears to have snapped off. The visible threads seem 'clean', as if they have had nuts driven on them relatively recently. The plate shows signs of washers having been in place.
I would find it very, very difficult to believe that there should not be washers and nuts on all those bolts, or that they were there and somehow wound themselves off after the propellor departed the engine.
Hopefully the ATSB report will shed some light on whether this is normal.