Two possibilities concerning the mysterious "Burlap" and "Crackle" textures based on my extremely limited molding experiences:
Possibly Ken reinforced his actual Plug with the burlap and it had nothing to do with the mold at all? He could have pressed the burlap too far into his skin coat of Ultracal and the imprint transferred to the surface?
As for the crackle, I know that when plaster bandages can simulate this crackly look sometimes when they dry. Maybe these were used in some capacity and the texture transferred.
Just some of my guesses.