rain gutter compound

[Shrek lives]

I am in the process of restoring a 64' F250 high boy. I stripped the cab down to the bare metal and removed the material from the rain gutters around the cab. Do any of you have ideas as to what the material is? It is close to 50 years old and is very brittle. Was it at one time a pliable substance that was used? Sort of at a loss on this one as what to replace it with. Bondo? Silicone? Appreciate any direction from the crew. Thanks