Ken's theory about a shortage of panels sounds feasible, although when the panels arrive, all the cars will have to be carried back inside & placed back where they were removed from the assembly line- why wouldn't they put the wheels etc on & add the panels 'off assembly line' when they turn up?
I also assume that the engine must be in the car, otherwise, why bother covering the engine bay?
It's a pity the photo caption in the archive didn't have more information! At least it gives us lots of room for conjecture.