I wish I personally knew more but I don't. All I can say is that he said it's preformed into a bell shape. He's not a newbie at working on these old trucks so I don't think he's missing the boat. He just said they are preformed cork and that the cork gets worn by the movement of the truck's suspension flexing etc. I can understand that being the case and the soft cork getting torn or worn thru.
I did search thru some of my catalogs online and didn't find any thing that I thought looked like what he said. I know a guy in Oregon who resto's these...likely he has a source. Dr. J
who is part owner of The Filling Station