You're right, it's the factory invoice that was kept on file by Ford. According to Marti, Lois Eminger used to work for Ford and when she learned that these documents were about to be destroyed by the company, she asked for, and was allowed to keep them (I'm not sure she had to pay Ford for them). She eventually sold them to Marti, who makes the original document available to those of us that can prove ownership of our particualr Mustang. I think it's great to have a something that accurately documents what your car did/didn't have when it rolled out of the factory. Marti has a list of the invoices that are available.
Here's the link to the Eminger story at the Marti site:
Marti Autoworks - Concourse Quality, Hobbyist Price
All the best!
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1965 Convertible 200 I-6, C-4 Auto (originally 3-speed manual) "Groovy Red" (originally Wimbleton White, then dark green) Newly restored and finally home!
1969 Mach 1 351 (W) 3-Speed Manual, Winter Blue, Deluxe Interior, All original, per Marti Report/Eminger Invoice. Finally moving her from the barn to the garage to begin work!