It is what the name says, a sunken date wheel. Due to the constuction of the movement with the min counter on 3 position, the datewheel sits lower then on the gen. The modification to transfer the min counter from 12 to 3 position is done above the date wheel.
Movement on the left shows the transfer gears, movement on the right is gen Eta 7750, Can you see the sunken date wheel now ??