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

Jim Lusk said

The trip odometer on the 120 speedo was 1967 only (as far as 67-69, can't remember on '66).

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I hate to say never since there was a lot of oddball stuff done back in the day, but, I've never seen a '66 Barracuda without a trip odometer......whether it was a 120, 150, tach type, etc.