Dave,
What you outline sounds correct for what I have found. They are a RARE option.
Shoulder belts were introduced in the second half (late Jan./early Feb.) of the 1966 production run. Also, I found that they were available March, 1996. If your car had been built later and did have mounting points for shoulder belts you would have had a small 3/16" hole punched in your cowl tag to the left of "BODY BY FISHER", at the bottom of the tag. I'm attaching a picture of a 1966 Chevelle tag that has the hole as an example.
Beginning around the 1st or second week of January 1966 a single small hole was punched into the tag, generally around the lower left corner, to indicate the car was equipped with anchor locations for the optional shoulder belt. They used clips to keep the belt secured when not in use. Sort of went to the A-pillar and back. The mounting holes are near the coat hangers, not sure about a convertible, must come from the rear seat area.
Here's a pic of the hole location under the headliner on a 67 GTO. The first hole is about 1" ahead of the coat hook and the second is 2" on center with the first. They appear to be 1/4x20.
Here is a photo from a Caprice.
Eckler's sells them as a kit. Perhaps they are not total correct. It talks about drilling two additional holes on 66-72 Convertibles. Click
HERE .