I had just started looking at 6-piece burr designs and computer analysis
of interlocking puzzles when Peter sent me this design. It requires 9 moves
to remove the first piece, and there is only this one way to assemble the
pieces into a 6-piece burr. Before this discovery,
the highest unique level I was aware of was level-7. The design uses a
back-and-forth motion to achieve the high level, hence the name.
