There's no official "K-code" designation for a Ford Mustang. The letter codes used by Ford to identify Mustang engines and options varied by year. While some enthusiasts might use "K-code" informally to refer to a specific engine or option package, it's not a factory designation.
You're likely thinking of a specific engine code from a particular Mustang model year, but without more context, it's impossible to say which one. To identify what someone means by "K-code Mustang," you need more information, such as the model year.
