Success!. I completed the moulds - mainly just applying and sanding Bondo to close gaps and smooth the interior of the mould - and then cast two counter weights. The results are shown in the pictures - along with some detailed pictures of the mould itself.
The completed weights are a very close match to the desired shape of the weights shown on page 9-15. Comparing overall diwidth and thickness of the cast moulds to the counterweight pieces supplied with the cits shows that the cast counterweights are larger that the supplied pieces by about 0.004" in thickness and width. The surface finish of my cast wieghts is significantly better than that supplied, and there are no visible voids. I am very happy with this result.
The light grey visible on the moulds is gre primer paint that I applied to teh mould. The idea was to try to seal and harden the MDF surface. Didn't work. The heat of the lead caused the paint to blister and crack, degrading the surface finish of the cast lead. Image three shows the mould after two pours - the paint has almost entirely burned away, but the MDF is stll in good condition.
The next step here will be to add a filler in the mould to delete the raised portion on the side of the counterweight, and the cast the two next two weights. I'll then trim the sprues from all four weights.
