I owned an A code 65 Fastback with a factory installed 9".
Short version, I bought it from the original owner in Dallas. He bought 5+ Fords per year. Among the cars he bought were a matched pair of 1965 GT's, a convertible and a fastback. Both were special ordered, and among the items on the special order were they were painted the Thunderbird blue green and they bought had locking 9" rear ends. He had a book with all the service records and the special order paperwork with the correct code for the 9".
The reason for it was both of them, but primarily the fastback, was used to haul a rather large boat, and it had to back down the ramp to launch it. Ramps can be slippery due to the plant growth so he wanted a heavy duty suspension, 9" locking, etc.
Sadly, cars were like paper cups to him, and the fastback was HEAVILY rusted in the rear end (rare for a Dallas car to be rusted) from being dunked in the lakes and not properly cleaned. I played with the car for a bit, then scrapped it.
Many years ago I posted on VMF a picture of the build and order sheets, showing the custom order on the paint, and 9", among other things.
When you buy lots of cars every year, I would posit that the dealer would cater to your special order whims.