How can a 4 piece puzzle be so difficult?? All you have to do is insert them into the frame…    It is a Stewart Coffin design (#217) originally called “Four Fit” but renamed by Stewart after Martin Gardner’s week long efforts to solve it were in vain.  Martin Gardner (1914-2010) is considered the Father of Recreational Math with perhaps the greatest puzzle mind of the 20th century.  Martin called it “The finest dissection puzzle of all time”. 

The 1/4" thick pieces are precision laser cut from 4 different hardwoods.  The base and cover measure about 6” square.