It goes in the vent tube from the oil tank to the intake. It was not used on early cars like my '73 or your '74, but in later ones. I believe from 81 on. It offers a restriction in the pipe that reduces air flow so the engines do not stall when the oil cap is removed. I would think it also prevents excess oil from being sucked into the intake.
It is part 10a in the diagram from the PET for SC's.
