In a heavy closed-shed dobby of a weaving machine having oscillating levers which are selectively connected to one of two oppositely movable actuation knives by rocking double hooks mounted thereto so as to control the movement of heddle frames to which the levers are connected, a pair of locking members pivotally mounted to each lever. An elastic element is provided to urge one end of each locking member toward abutting engagement with a double hook thereby preventing pivotable movement of the double hook when engaged by one of the knives. The locking members are released from engagement with the double hook upon simultaneous engagement of the knives with a lever.