A major difference in the interior was the Series III had dials to control the heating and ventilation, big and probably Bakelite dials, whereas the Series IV had chromed slider controls.
The controls were the usual heat or fresh air and demist or floor level.If the air flow was inadequate, you could turn on the blower, in modern parlance, the fan. Just one speed mind. The blower was either on or off.
On cold mornings when going to work I would at times turn on the blower to give a bit of extra heat. All was good, until I stopped at traffic lights and the blower would fill the car with engine fumes. But worse was if the weather was damp. As soon as the car stopped when the blower was on, the whole car would fog up. It did not have air conditioning to clear the screens immediately so fogged up was a nuisance. I learned to always turn the blower off while stationary.
A Humber also featured in the ABC TV documentary called The Nationals (Party), once known as the Country Party. I did not see the outside of the car but I immediately recognised the rear seat reading lamps of the car containing I think former Country Party leader the late Jack McCewen.
At times I miss my Humber. What I don't miss is that my current car sits on 6.7 litres of fuel per 100 km, 42 miles per gallon, and the Humber used to sit on 18.0 litres per 100 km, 16 miles per gallon, but this figure dropped off considerably when it was cruising at highway speed.
This is pretty much like what my Humber looked like, without the mudguard mirrors. There was an exterior mirror next to the driver's window. Note the plug in the bumper bar that could be pulled out and the crank handle inserted. Yes, more than once I crank started it.
This is the dashboard of the Series III, with quite nice Bakelite controls.