Would the 1111 be for the Aniversary Gold, since it was not a regular Mustang production color.
That (with a two digit district number on the front) would have been the special order number of a group of 66 (in this example) Anniversary Gold Mustangs it appears but if you had another special order number (for example 561111) from another district that likely identified a taxi cab or a 1 ton truck painted John Deere green (just an example).
Now if a Mustang buyer in Buffalo didn't like the RPO colors and choose the same color from the Fleet Color book it would have been built without a color code on the door tag and a different (example 370158) DSO code. Each of these cars was suppose to get an envelope in the glove box with a copy of the special order paperwork to explain to the servicing dealership what the non-RPO item was on the car.
I've owned more than a half dozen of these special order cars - currently a special order paint 69 CJ Mach I
The first book is dead wrong on the anniversary gold thing. The confusion lies with the special order code of 1111.
"In 1966, a rumored 50 special ordered Anniversary Gold Mustangs were built as part of the Millionth Mustang Anniversary, special order number 1111 and sent to dealerships across the nation........."
Wonder what the rest of the DSO was on those cars - guess I could look it up.
In any case you might have a point considering the understanding of things in the 80's is not what it is today. IMHO it was be a heck of a coincidence to have two groups of cars built two years apart sharing the same special order number (well at least the last half) and the same exterior color.