This key is a fairly common antique pocket door key made of brass. This key utilizes a screw above the bit in the shaft to adjust the length of the key. The screw turns freely and uses pressure on the inner shaft portion that is connected to the bar shaped top to lock the length in place. Although it is hard to see in the pictures, this key does have a unique design element. It features a darker color diamond in the middle of the bar on top.Image showing screw:Image showing reverse of key:Image showing diamond on top: