KV is right, that second link takes you to the current home of Castings, Inc., which is the descendant of the old Rappaport Bros. from Chicago, of Home Foundry and Junior Caster fame. They definitely had a shell burst in one of their molds. It would be worth looking through their online catalog, though unfortunately, there's nothing so easy as a "Shellburts mold" link on the page, you'll need to drill down.
And I'll second KV on his other suggestion: Check eBay, for old Home Foundry or Junior Caster molds, or even the German manufacturers like Schneider Bros. or Ideal (whose molds the Rappaports copied, when they first started in business). They all made shellbursts of one kind or another.
In fact, as of last night, there were several auctions in the toy soldier-pre 1970 category on eBay, and I think they were due to end in the next couple of days. Also, they were relatively low-priced at that time, $10 or less, which is a steal for metal casting molds.
Hope that helps, prosit!
Brad