Post #5 on the attached thread shows the same type recirculating (economy) heater used in my 1960 GMC.
http://www.67-72chevytrucks.com/vboa...d.php?t=373970
And here’s another thread showing how they look from the firewall side – posts #5 & 6.
http://67-72chevytrucks.com/vboard/s...ghlight=heater