Filler Material inbetween barrels

[B. Dudley]

I recently took apart a couple of sets of junk Parker barrels. One set had a tube blown out and the other set was cut short and badly pitted.

The one set was Parker Special steel from a 200,000+ serial Meriden gun. It had nothing between the tubes but 3 metal spacers that did not appear to be soldered in.
But, the other set was a earlier 10g. hammer gun top action set. When I removed the ribs, I found that the space between was fully packed solid with what looked like dirt or something. When I cleaned it all out, there was no serious pitting on the surface of the tubes, so it was not rust, besides, there would not be that much rust if it were.
Does anyone have any idea what this might have been? Did Parker pack the barrels with something to prevent corrosion? One Gunsmith I mentioned it to thought maybe cosmoline as he said it can dry out a look like dirt with heat and time.