The effect is sometimes called the China Computer.  The three blocks with numbers 1-2-3 on the front are stacked in various ways but always rearrange magically in 1-2-3 order.  The blocks are self-contained and mechanically clever as no shells or external pieces are used. Thomas created a number of different versions of this effect over the years and this is probably the hardest one to find, with much smaller blocks in a beautiful contrasting natural wood finish.