[Arran]

That radio is most certainly from the mid 1930s or later, the earlier car radios used a back lit celluloid/phenolic plastic dial viewed through a peak hole and used brown or black Bakelite knobs. The earliest car radios were basically like a farm battery set with "A", "B", and "C" batteries, and they were almost never installed by the car manufacturer. Philco produced many aftermarket car radios as well and in something like a Duesenberg, which often had custom built bodies, they could have installed any off the shelf car radio the customer wanted. That head unit looks familiar for some reason, I may have seen one in an old catalog or service manual.
Regards
Arran