That, my friends, is a Predator variable venturi carburetor on a Z-car. It is similar in function to the SU but in a downdraft. It uses two "air doors" above the throttle plates that open up with the airflow and regulate the amount of fuel the engine sees. There is a wing shaped fuel bar in the "shadow" of these doors that forms the venturi and draws the fuel from the bowl. I wanted to try one on one of my cars but since they were designed for big honkin' V8s, I figured it would be too much carb. Besides, I think they are a bit pricey too.