Abstract: A mechanically actuated downhole locking sub is capable of locating and locking into a doubly tapered downhole landing nipple adapted to a magnetically coded, electrically actuated lock. The sub permits the substitution of mechanical actuation for electrical or explosive actuation within a drillstring or a wireline run for setting and releasing downhole tools within an oilwell without requiring a changeout of the landing nipples, which may be set in the casing or be otherwise unremovable. The mechanism includes a specific spring biased split locking ring design which permits the mechanical locks of the tool to be set in a cocked position downhole and then to be positively locked upon engagement with a landing nipple where the tool is raised. An internal engagement mechanism, mechanically connected to the fishneck of the tool, permits the locking ring to be disengaged subsequent to landing and locking the tool, thereby cocking it for removal.