Prior to landing, the landing gear was selected down and the pilot reported that he had normal down and locked indications including illumination of the single green light, the visual arrow indicating down and the nosewheel visually checked down via an external mirror. On the landing roll the left wing lowered until the propeller and wingtip hit the runway. After the aircraft came to a stop, the pilot got out to find the left main gear had not extended. Investigation revealed that on this particular aircraft model, the single green light that indicates the landing gear is down and locked, is activated by a switch which closes when the actuator has moved to its full extent of travel. Further investigation revealed that the actuator arm linking the actuator to the left main gear was bent. This prevented the arm from extending the left main gear. The reason the actuator arm was bent was not determined.