this one is an arak, and may be somewhat similar as to assembly of the stop mechanism. mine has push buttons, but all they do is displace a pin that presses on the outside of the balance wheel or releases it. it is a very simple and straightforeward flat spring action that simply screws onto the back of the mechanism and is completely independant of the rest of the works. the buttons are put in place and are held there by the presence of the whole mechanism.
the arm visible on the right is what swivels, and it pivots around the big screw that holds it in place. the pin that presses against the balance is at the end. can't see it as it's pointing down. the end of the flat spring can be seen overlapping the arm. i've never had the opportunity (luck) to see the innerds of a sh. hope this can be of some help. mine was simple to take apart and the reverse, except holding the pushers in while putting the mechanism in was a pain, but not too complicated. can i get a pic of the sh works here? just for info, the start/stop mechanism on my junghans stopwatch works in the same manner. tim