4 sets of Lego Bayer Test Bricks from the early 1960's in blue, green, earth orange, yellow as shown.

Each set of 4 bricks in 'A/B/C/D' tests the clutch power of Lego bricks. 'A' has the weakest clutch while D is strongest clutch. Lego chose C for all bricks ever since. 

These are rare prototype bricks in played condition, see pictures.