Crankcase ventilation.

It’s a 600 CFM vacuum secondary carb.

Take the hose that goes to the power brake booster off the carb. Remove the pipe plug in the intake runner and hook the brake booster up there.

Hook your PCV valve to the port on the side of the carb. You want the PCV hooked to the carb, not the brake booster. The carb is calibrated for a controlled leak at that port and the PCV does that.

I almost never hook the PCV up to the manifold. It should always go to the carb. There are exceptions, especially with aftermarket carbs because some guys think a PCV is an emissions part (it is) but it is also a critical piece of the tune up picture of the carb.