I don't ever remember seeing much of any original floor pans remaining, on any of the 30s or 40s Mopars I've had, much less what might have been covering them.

There were a few places where I've encountered some type of undercoating that looked a lot like that stuff. I suspected it was originally factory installed. It was some good stuff! I've seen clean shinny steel under it, next to rotted away sheet metal that was not protected by the undercoating. If only the entire floor would have been covered, the sheet metal might have still been good. Gene