The
winglets attached to the rear fenders effectively help to generate rear
downforce and are used to tune the car's aero. balance. For high-speed
circuits such as Le Mans they are removed to reduce overall drag. 
The
rear wing runs at maximum height.
The
Reynard's rear diffuser is not to the maximum allowable width, which
was common practice, in order to reduce the spinning rear wheel's
negative interaction with the
diffuser. But the most interesting treatment is the curving
transition where the fender underside blends to the diffuser side
wall.
Without winglets here.