Sound Deadening What is it?

I am restoring a 74' 365 GT4 2+2. I am redoing the fuel tanks. What is the heavy light brown material sprayed on the tanks. There are a areas sprayed in the inside of the rear body panels as well. I have seen what I believe to be the same material on a 308 QV so it has been in use for a while. Does anyone know what it is and who makes it? I posted this in the vintage section, but thought this section might be more appropriate. Thank you in advance.