I checked my Glock 17 last night. It has only one screw and it adjusts elevation. My Glock 35 has a competion rear sight like the one in your picture. The rear screw adjusts for elevation. When facing the slide from the ejection port side, clockwise will lower impact. The forward screw is windage, clockwise brings impact to the right. Be aware that there no "clicks", adjustment is a smooth movement. El Gato is correct the sights are too fragile for carry, but they are great for target shooting.
