From Hoffmann's 1893 book Puzzles Old and New, via Martin Gardner's Scientific American column sometime in the late 70s/early 80s, via my childhood memory, via Blender. I made many of these in balsa wood as a kid. Not very hard by the now-well-known analysis. Disassembly video is at: http://youtu.be/8Efjz7pOQqUThe STL files have 0.01" extra clearance; this has a good fit on 3D Systems' Cube printer. The Blender files for both this "relaxed fit" and the ideal-width pieces are provided. Pieces E and F should be rotated to lie flat before printing, of course.