The MR B3RB car should look like this in the pre-production unit after opening the box.
Proceed with attaching the side parts of the car.
Re-use the screws and locknuts that held the top plate to the side metal, that you took out previously
UPDATING: The production units will include the larger side plastic skirts instead of the small plastic clips shown below. It will no longer be required to remove the top plate screws and clips.
Once attached, the side components should look like this. To better illustrate the process, one side is shown attached while the other remains detached.
