139 Burr
Designed and made by Bill Cutler; can't be disassembled.
(Red Oak, 4.5 inches)
From his computer analysis, Cutler determined that 139 was the largest
number of states that a standard 6 piece burr could have without having
a solution, and then he chose the simplist of these for this puzzle. So
this burr is made to the dimensions of a normal 6-piece burr, has lots
of movement, but can't be disassembled (it was made by gluing two
portions of a piece together during assembly).
Here is his diagram of the pieces: