That was
eccentric double flare tool.
Double flare feature not needed for the rivets; however the simple flare tools are often not eccentric spinner type which is desired.
My notion is to miscegenate the screw and eccentric needle bearing cone from one of these:
View attachment 118456
with one of these heavy I-beam clamps:
View attachment 118457
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& spin grind the forming cone to necessary shape, make tail end anvil & spin to shape on a surface or T & C grinder as well.
Mine is a Rigid, however these tools are available in a range of prices depending on source, including "import", or surplus or fleamarkets, etc. The clamp body could simply be cut out of, say 1-1/4" plate or whatever will accommodate (thick enough) the given screw from the donor.
Depending on rivet size, the cheap spinners from china for soft metals might be contrived to work with a suitable shop made or shaped & hardened cone (I don't know). The essential feature is that it is eccentric, and spin-forms the shape, not just compress it. (Though simple compression might or might not be adequate)
Making the tool does not seem inaccessible nor like it needs be particularly expensive.
However, there are a lot of ferules, crush tubes, and hollow rivets to be made.....
Along with a bit of engineering as you mention.
smt