Meet the click spring, also know as my problem child. Something that is smaller than a hair, but as strong as a staple. I have currently lost two of these in rebuilding the SW200 movement. On slight shake of the hand while inserting it into the main plate, and it springs out faster than the eye can catch. And there is no way to spot this thing easily when it does.  

Three more are ordered, as well as a pair of anti-magnetic Swiss made needle point tweezers…