HABA's Sorting Box Plug & Explore has it all!  
The coloured boards can be slid in with all sorts of combinations.  The opening that is created, however, means that only one block will every be able to pass through.  Here little explorers can have many goes at playing with the various shapes.
 
Contents:     1 sorting box with 6 sliding boards & 6 various shaped blocks.
Material:       Beech wood.
Dimensions:  Sorting box L25 x W10 x H12cm
 
Age:   1+