I figger they painted the whole cab interior & firewall fawn, then added the body color @some plants-if you look at a bare cab it would be easy to drape off the firewall. I also theorize they painted two tone truck cabs white, then added the lower color-I have seen firewall markings indicating "ivory" or "two tone" on many trucks, also found white paint under the body color.
I'm sure things were different from plant to plant-most of mine have been St Louis built trucks, but the burb is from Fremont-I notice a big difference in spot welds, both the number of welds & quality.
over @the 67-72 board there are pics of many fawn firewall 60-66 trucks, i have only seen one IRL though.
