There is a standard way to adjust levers on governor shafts, as you know, and yours looks as if it conforms to that process (just because it has a screwdriver slot in the end of the shaft, and a clamp bolt on the arm). Also, in addition to applying for example to most Honda engines, that process is used on the Villiers 15 series: see page 10 of the owner manual:
http://www.g1jbg.co.uk/autoculto/pdf/villiers15.pdfIt is possible that your governor shaft rotates through 360 degrees because something has gone wrong inside the crankcase, but before taking it apart, it would be way better if one of the Villiers guys on Outdoorking joins this thread. I know Joe Carroll collects Villiers engines, and has done a detailed overhaul on a Mk 12, which may be a close relative of the 412 (I don't really understand their model numbers).
