That sure is a well worn 13! I think you are on the right track, that front screw is what secures the yoke. If I tighten that screw too much on my Model 19, the cylinder will not open. If the screw is too loose or removed the cylinder will open but will come out if the revolver is held at an angle. Perhaps the wrong screw is in ,or is missing. In that case a new screw will have her working great. As well as a Wolff spring kit - makes an old revolver feel brand new!