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

All I can tell you as I worked in the Detroit/Hamtramck area when these were new and owned quite of few barracuda S's and worked on a lot of them for customers, and from 66 to 69 I never seen a 120 speedo on any of them. But as I said before I seen a lot of variations as the assemble lines would substitute parts not to shut the line down. I do remember my 68 had the trip my 66 no trip and I just can't recall on my 67 or 69. Hopes it helps.