Man, I have a '35 PJ and I have exactly the same question. It's a seemingly simple little thing, it seems to be twisted oily-type paper that is compressed into the channel and allows stuff to be tacked to it. It has to be removed to properly clean up the body, and it mostly disintegrates when it's removed. I can find kraft-paper type stuff for wicker chairs, and I've found some stuff which is rubber or some type of soft plastic material. That seems to be what more modern vehicles use. Just seems like someone would have a source for the original stuff. In my PJ, it's about 1/2" square.