Using a flexible spring-steel plate above the spark plug as an engine stop control is a standard technique - Briggs and Stratton were doing it by the beginning of WW2, and perhaps earlier. It just short-circuits the magneto secondary. Unfortunately depending on the quality of the earthing of the spring steel, you tend to get a high-tension electric shock when you use it, which is annoying. Note that it does not prevent the engine from re-starting, so it does not meet modern safety standards.
Not sure yet about that screw and nut on the right side squeeze control. I noticed in the photo. It may be a stop that you adjust, setting how far the handle moves when you squeeze it. It may be original.
I haven't recognized the engine yet. It could be a traditional British one, or even a Briggs & Stratton, though it doesn't look like a B&S. Either way it will have some identification on it somewhere, and you'll find it when you clean it up. Also, ATCO mowers usually have a stamped brass plate on the outside of the right side-plate, giving model number and perhaps serial number.
