What’s the exact process to identify a real 66 Formula S Barracuda?

If you had them that long they probably did come from the factory that way. I worked in a gas station in the 60’s and 70’s right between Dodge main in Hamtramck and the Lynch rd assembly plants. And litterally a 1/4 mile from where all the 68 Hemi dart and cuda’s were parked along the fence to be picked up. There were a lot of variations from what was suppose to be. When the line ran out of something and they had a item that worked in went in the vehicle. But most of the formula S that I seen back in the day had the 150 speedo.