Calibron 12

The objective is to fit all the  pieces in the large square opening. There is only 1 solution and it is very difficult to find.  We packaged the puzzle unsolved, but the solution is provided.  In the frame I made an extra space to hold one of the pieces so that we could package it unsolved.  Our version is made from precision cut woods with at least 4 different woods in each puzzle. Ash, Maple, Walnut, Oak, Mahogany, Birch, Cherry, Alder, and other woods may be used with each puzzle have a little unique look.  The pieces are very strong at 1/4" thick and have a nice finish. 

One of the many things I like about this puzzle is that it does not look nearly as difficult as it actually is.  People seem to get absorbed in it quite readily as you always seem to get close.  It fact though, it is a difficult level 5 (of 5) puzzle.

Originally designed in 1933 by Theodore Edison, son of the famous inventor Thomas Edison, and made by his company Calibron Products of West Orange, N.J.   It was offered as just pieces and you were told they made a rectangle and there was only 1 solution.  Turns out the only possible solution is a square shape, as verified by Ken Irvine's computer analysis. Ken is working on a full analysis of the puzzle, with excerpts noted below.  I will link that analysis here when it is available.

From Ken Irvine:
After determining that there was only 1 solution to the square (using Burr Tools program), the second goal was to determine if there were any additional rectangles that could be made with the 12 pieces. The first step of this analysis was to determine how many possible rectangles needed to be checked. Since the total area of