What does the 2 plus mean on 1965 ford mustang fastback?

The "2+" on a 1965 Ford Mustang fastback doesn't refer to a specific factory designation or package. There's no official Ford terminology using that notation for a 1965 Mustang. It's likely a later, aftermarket, or owner-applied designation, possibly referring to something like:

* A modified 289 cubic inch engine: The "2" might allude to a V8 engine, and the "+" indicates modifications or improvements beyond a standard 289.

* A 2+2 seating configuration: While not standard nomenclature, someone might use it to highlight the optional rear seats, which offered limited space.

* A custom designation: The owner might have added it to represent something specific to their car's modifications or history.

Without seeing the car or more context, it's impossible to say definitively what "2+" means on that specific Mustang.