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A fascinating 4-piece puzzle with 2 identical pieces and 2 identical mirror images which interlocks into a tetrahedron. This is the metal version of "Stan's Tetrahedron" at https://www.shapeways.com/model/53194/stan_s_tetrahedron.html It fits nicely out of the box, although the fit is a bit looser than in the plastic (WSF) version of the same puzzle. Update Mar 15 2010: the new version has no holes visible when assembled. Sphere diameter: 12mm.
Only a few copies of this puzzle have been printed. It is quite expensive and I am not sure if the tolerances in metal are good enough to reproduce it reliably.