Dont know anything about Smith hammer guns but that one looks just like a hammerless. If it is and you havent found it, there is a screw under the top lever that screws into the same hole, from the top, that the trigger guard is screwed into.
That hole in front of the trigger guard is the bottom of the top lever shaft, it has a very heavy spring pushing against it. Tap on it with a punch in the bottom of the shaft hole so you dont damage threads and move it only far enough to disengage the trigger plate. That spring will give you lots of grief on reassembly. There is a special tool you can make to make assembly somewhat easier.