There is a small air bleed hole in each side (hidden from view in the pics) just under the rear baffle in each corner to vent the tops of the kick-outs.
When I built my first pan I did expect that the extended sides design could possibly create a trapped air pocket in the tops of the kick-outs. But when I finished welding & filled the pan with water to check for leaks this turned out not to be true. The water filled both the open sump area & the kick-outs behind the sides equally....can't explain why but it did.
But even though the water filled all sections equally and appeared not to create any trapped air pockets, I was still worried it could still happen with oil since it's thicker than water. So I added the 2 air bleed holes under the rear baffle anyway just to be safe.