Stewart Coffin Design Wood Puzzles

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”.