Mind if I drop into this conversation for a moment
As a plumber of the old school, I have poured many a lead joint to adjoin cast iron pipe fittings to pipe ends. Actually did two joints the other day, first time in a long time. Anyway, when lead is melted and is in it's liquid form, it is as liquid and fluid as water. Also, it is smoking hot. If pieces of pasteboard or cardboard were just stuck in-between the fins, unless they were oil soaked, they would burn up and the lead would not be contained. Plus, the density of the lead makes a lot of weight and I would think even if the pasteboard was oil soaked to keep it from burning and then stuck between the fins, it would push out due to the weight of the lead entering the area, unless some intricate device inside was holding them in place. I looked at my James with the sabot, and the fins are tapered, but in the wrong way to accept and remove the pasteboard after the lead would be poured. Plus, they are contoured and rounded, not easy to plug with a thick pasteboard. Nor was the lead close to the fin tapers inside. Then, like I started this post saying, the lead is not thick like honey or anything, it is like water. So any minute hole or opening between the pasteboard and the iron would let the lead just run right out resulting in a bad pour. And the worker would have to melt off what lead adhered to the iron and start over. A pain in the ass. The skill level to do all this would have to be very high... but then again., it boggles my mind the intricate casting of the James shell anyway..
If this drawn illustration and verbiage posted previously above wasn't in existence, I would of offered two other ways it could have been done;
Firstly, the easiest way to attach the lead would to be to not pour it at all. Sheet lead could have been cut to the perfect size, and just pressed in to the fins from the out side. Super simple, and I still like this idea, except for the lead in the 4 holes in the bottom, I can't explain that. . [ by the way, it would be extremely difficult to pour lead into those into those tiny little holes to fill the sabot area.] Plus, unless the cast iron of the shell itself was heated to a couple hundred degrees, liquid lead resorts back to it's solid form almost instantly, in one or two seconds. You have to work extremely fast with lead.
The second way I would approach this, if you want to believe the pour theory, would be to fill the whole inside of the finished cast iron shell birdcage area with casting sand, and fill it into the fin area also. One solid chunk of casting sand. . Then scrape away what area you want the liquid lead to go to adhere to and fill. Of course, there would be a 'sleeve' on the outside to contain the liquid lead. After the pour break up the casting sand inside to expose the cavity.
Anyway, this is an interesting subject. Hope we find it out eventually