In working on the truck today, removed the rear fenders and was able to get more into the side panel that is attached, sure looks like they used some form of body compound, as they filled and smothed over all the rough spots, and there are blops of the compound here and there. there are NO nails or tacks along the top seem, so it was glues/ adhered by some form. My bet is they used a body compound the more i dig into it. There are tacks along the seams on the outside.
I have scanned those phots many times to help me rebuilt this truck.
