This is the alloy Lock Ring that holds the front sprocket spider onto the motor drive pinion on the Bosch Classic+ Drive Unit.
Remember its reverse thread and only needs to be tightened to 30Nm.
We also sell the Bosch Classic+ Spider Tool to safely remove and replace this nut. Available from our store.
Just to confirm its to fit the following motor shape. If you require a Performance lockring, you can find it here Bosch Active & Performance Lock Ring.