Is it some kind of spray foam?

[jumpsoffrock]

I believe they simply used the same seam sealer as the rest of the parts of the cab. The application was generous; similar to how they did it on my '71 Camaro throughout the cab and even on my '56 F100 in it's cab corners. All vehicles seam sealers are very fragile, and am sure that is due to 1) their initial composition, and 2) age(obviously). The Fords corners were full of loose chunks, and so is the rotten GMC cab I'm removing for my project.

I have two '72 cabs that appear to be original and I'll take pictures today, since one of my chores today happens to be cleaning out my new cab.