Williams: this inlet blanking panel (arrowed) reduces the cooling airflow to the radiators
One last development spotted from Jerez was this inlet stuffer on the Williams. Normally teams add tape or blanking plates over the radiator itself to tune cooling to the ambient conditions. This more sophisticated solution suggests the Cosworth does not need the full cooling provided by the initial FW32 spec. So this stuffer not only reduces the cooling airflow to the radiator but also imroves the airflow around the sidepods, reducing drag.
Scarbsf1's Blog 
Nice, looking at how the Lotus is kind of bulky I guess we could be expecting some dramatic shrinkage and performance gains when they do a new aero package too?