No idea why it still had seats here is a bit of background information that might help.
Since the whole car would have been used to carry parcels there would have been heaps of room, remembering that that the blue spot parcel service acted like a courier service between stations. I could in fact see that the seats could have been handy with each stations having its own seat on the train like a traveling post office. The seats in use at this time were like wooden church pews. the items carried would have been what is now called by Australian Post as small packets. Remembering some items may have only be going as far as the next station up the line.
The May 2013 issue of the Australian Railway History magazine reports that's passenger joining the parcel service at stations stops was indeed a bit of a problem,so it must have looked like a normal passenger service and hence the adding the parcel express signs to the front and rear of the assigned carriages.
At first the signs were added to the front and rear of the assigned cars but the crew found it hard to move between carriages so they then just used them on the front and rear of the train. D4001's parcel service was just a temp assignment, that lasted about 50 years. As a driving trailer she still had to be attached to a power car not having Panto or traction motors of her own. This would have been what ever driving motor was free to do the duty at the time.
And yes often Passenger and Parcels were carried on the same service but not in the same car.
