it's a Tar like substance used as caulking to hold the tubing in place, (it's was called bitumen), it has some minor insulating quality, but mostly it was the "hot-glue" of it's day, later they, (Ideal Corp) stopped using it & switched to soldering the lines in place to secure them to the tank, on some late production models they finally were using spot-welded brackets that were bent around the tubing to fasten it to the tanks walls ( think of this method as metal tie-wraps)
Fun stuff...
