That is Ziebart undercoating. Will be interesting to see pics of the underside. Some customers had the undercoating sprayed inside the trunk and also higher up on the inner fender panels under the hood. Just makes for a MESSY car to work on.
VERY COOL CAR. Are you sure about the disc brakes? Master looks like a drum brake one. If the wheels are BBP it's disc. SBP and it's drums. Slant cars without air had drums and SBP thru '76.