I get it from my local camping supply house (on a 20 ft roll). It was probably the remains of some D-19 tape that VW and Karmann used for keeping out water.
Does anyone know if this came originally, or maybe was put there by a previous owner somewhere in the distant past? I found a bunch of old chalky caulking material between my channels and the body metal.
Now where the heck were you six months ago, when I was struggling to figure out how this all worked? Awesome post, and thanks for including such great pictures!