What is this screw for?

The vertical screw 2” forward of the pump shot. What does it do?

It adjusts the secondary throttle blades. On a standard holley, the screw it down under the throttle plate on the bottom side, but aftermarket or hopped up Holley type carbs usually include the screw on top to be able to adjust secondary air control from the top. Alot of times you use the secondary throttle blades to add a little more air to increase idle on a bigger cam if your primaries are adjusted correctly using the idle transfer slot.

sets secondary throttle plates. if i see that right