This is wood window sash balance for a 6 lb. window or 12 lbs. if one is used on both sides. This sash balance style has been used by many manufacturers of windows for several decades. It fits a 3-1/4 in. by 6-3/8 in. pocket that has been mortised about 2 in. above the centerline of the window frame. It features an all steel construction, with a coil spring inside that retracts aircraft cable lifters as it offsets the weight of the window (vent).