Mike the lock plate connecting screw (long one)looks ok and so does the one under the top lever. I have seen where the long lock plate connecting screw was the culprit in the trigger not working.
Have you ever taken the locks off? If so you can look through the one hole and see the sear and see how the trigger plate is close or even resting on the sear. If the gun works with snap caps then the problem could be recoil that is causing a double fire and that top screw might be one turn too tight pulling the trigger plate up too close to the sear/s.