In June 1958, just 28 units into production (car # 528), the bracket for positive filter location was added to the Twin Cam air cleaner assembly. That is the rectangular plate at the top which is intended to keep the filter element in the outermost position and keep it from being sucked into the carburetor intake.

In September 1958 at Twin Cam car number 2469, less than 150 units from end of production, the "venturi" was added. This is essentially a short velocity stack incorporated into the base plate of the air cleaner, seen in these pictures. The effectiveness of this part is a bit suspect because of the squared off shape at the entrance end. My best guess without actual dyno testing might be about +1 HP at the rear wheels (although it could even be zero or negative). Click for more information on velocity stacks.