Here's my baseplate. It looks like the idle port is fed from the primary side, I think Dwayne mentioned that. The secondary metering plate idle channel lines up the air bleed and into the baseplate as a slot above butterfly.

Maybe the diagram from the FSM for a 440. The manual says the diagrams for the 4160 for both 383 and 440 is generic. Just saw that mentioned in the manual. My bad