These are vintage Smith & Wesson Model 94 high-security handcuffs, designed with a unique tubular locking mechanism.Model History: Introduced in 1969, these cuffs were marketed to the U.S. Marshals Service and are no longer in production.Key Type: They require a specific "D" or "N" key to operate, which differs from standard handcuff keys.Functionality: The key is turned to engage the single or double lock and to unlock the mechanism