Help Identify Early 318 Single Plane Intake

FWIW-
While doing some maint. work on my '75 Swinger today, I checked it's (original) 318, and sure enough- it has the single plane manifold on it.
Now curious, I dug out an original 318 from a '73 Dart I parted out a couple years ago from under the bench in my shop- it has the dual plane.
Both engines are basically unmolested. I'm not seeing any kind of a pattern here, if there ever was one...