Only about 50 of the roughly 44,808 Mustang convertibles Ford built in 1967 carried the rare K-code 289 HiPo V8, and most of that engine’s already-tiny production run went straight to Carroll Shelby instead. This triple-black example from the Brothers Collection might be the only one wearing a black deluxe interior. Here’s why documented K-code cars carry such a premium, and how collectors prove one is real.