I just learned from Gregg that reverse lights didn't come standard on Mustangs until 1966

Some early Mustangs built before August of 1964 (1964.5 models) will have a couple of additional differences. The headlight extensions will have a beveled edge to match a similar extension known as the “stiffening skirt” on the hood.

The pre-August 1964 and 1965 Mustangs have a Ford Falcon instrument panel with a horizontal speedometer and “idiot lights” for warning functions, but the 1966 has a Mustang-unique panel with a round speedometer and various gauges. Most 1965 GT models will have the round-speedometer panel, but if you see the Falcon version, you’ll know it cannot be a 1966 under any circumstances.

1965 Mustangs have gas caps with three notches around the sides and a recessed Mustang logo. In 1966 Mustangs, there are several small ridges all the way around the cap and the Mustang logo protrudes out past the words “Ford” on the top and “Mustang” on the bottom. A 1966 GT will have “GT” on the gas cap, but a 1965 GT won’t.

there is a backup light bolt on kit that costs less than a 100 bucks 

https://www.cjponyparts.com/backup-light-kit-mustang-1965-1966/p/BLK1/

https://www.progressive.com/lifelanes/on-the-road/1965-vs-1966-mustang-differences

https://www.hagerty.com/media/car-profiles/1965-ford-mustang-still-americas-most-popular-classic-car/

Ford sold around 559,500 examples of the Mustang’s first year, and if you count the total extended model year (the Mustang debuted mid-season in April 1964) the number rises to an even-more-impressive 680,989. 

The 1965 outsold 2018 by six to one.

Ford reportedly took 22,000 orders on the first day