A rotatable control device including a body housing a control structure and having an external box; a control shaft journalled in the boss for rotation about an axis and having an internal portion connected to the control structure and an external portion extending outwardly from the boss; an operating handle having an inner end portion connected to the external portion of the control shaft, a gripping end portion for rotating the control structure between first and second positions and an intermediate portion; first and second locking structure circumferentially spaced on the body at locations corresponding to the positions of the intermediate portion of the handle when the control structure is in the first and second positions, respectively; a slider member slidably mounted on the intermediate portion of the handle for reciprocative movement between locked and unlocked positions; interlocking structure for cooperating with the first and second locking structure, when the handle is in the first and second position, respectively, and the slider member is in the locking position, to restrain rotational movement of the handle; and bias structure for biasing the slider member toward the locked position irrespective of the orientation of the handle.