Good one Jon!
As I mentioned above, I "really know much about them". All I can say is from handling them is they are quite dense; not porous like cast iron. They also have a melted look to them. I'm sure someone could make something or know of something that naturally occurs with this sort of look but I'm not aware. You can tell by the fineness of the rust that the quality of the metal is good. It would rust very slowly. In this case the owner bought them from a dealer who deals in that sort of thing and they were from a known site. And yes ones of this size I noticed when I was getting a Google education on them do go for pretty big bucks. I believe these two would bring between $1000 and $2000 each but, again, that is my guess just from what I saw on the internet.