Differix (1974)

Each position board is comprised of 9 illustrations of a single motive. Each of these illustrations differs from the others in only a minor detail or change of position. At the first glance all the motifs are so similar that players are obliged to scrutinize them closely in order to match the pictures on card correctly.
In this way children acquire a sound grasp of certain shapes and learn how to distinguish between different shapes.
Differix can be played solo, in groups, as a competitive family game, as part of pre-school training and in the kindergarten.

The latest edition of 2006 by Ravensburger features 4 different motives in noticeably increasing difficulty (an elephant, a juggling clown, a juggling frog and a tightrope dancing pig).