1928 Heater

Try an internet search with your nameplate data. There were a lot of aftermarket heaters in the past. Some were mounted in the rear floor and routed exhaust through a cast iron heat exchanger, as you described; the one originally on my Series 80 was made of sheet metal and fit over the exhaust manifold. Air flow from the engine fan blew through it and through a port hole cut in the firewall. The hole was closed with a steel slide when not needed. All I have left now is the port in the firewall. Interesting topic!

Suggest you search Ebay as there are often ads for these from various period magazines, offered for sale. There are a lot of different brands and models.