Intake manifold vacuum line connections…help!?




I’ve recently finished rebuilding my engine and transmission in my m278 e550 coupe and have run into a minor snag! I cannot recall what connects to the driver side (US) port on the back of the intake manifold! Pic below for reference. On passenger side, the centrifuge line connects to port. On top, the vacuum pump/brake booster line connects….so what goes in third port?!!
Bottom left corner black port


