Did you guys see this??
https://www.grautogallery.com/vehicles/11073/1968-mercury-cougar-gt-e?fbclid=IwAR1t2vvcSed6HK1AUjw19pI73AlJ8KZLSdPkFhCXQSJBu1KKL3Ag6Hq41eo
Yes, I love the color
Yes, it’s being discussed here. 
How could the 3.50 rear gear make this car 1of1 when all GT-E’s came standard with that gear?
Apparently there is another one with the identical options with a different differential. To parse cars into one of one requires careful dissemination of the items listed in that exact order. It’s all semantics. Desirability and originality make a car valuable. Being one of one is simply a selling feature of a Marti report, not the car itself.
There were 2 XR-7 GT-Es with 427/auto painted in gold with a gold interior, FM radio, and white vinyl top. Of those, one of them had the 3.50 non-locking rear.
You have to read closely. A future issue of ATSOTC will address this topic.
What did the other one have? A locker.
Marti will know for sure. Otherwise, the answer may be held with the registry.