See: http://phasmatisapparatus.blogspot.c...ce-manual.html the 3rd manual may be of help. Although the manuals are for rim set shutters and yours is a dial set shutter manufactures follow similar mechanical designs from one model to another. I do not think the spring goes on the delay gear train or the set dial. A lever where a spring goes usually has a notch that the spring sits in but not always and the case will frequently have a grove that the spring end lays in. The fact that you say it popped off when operating the shutter with the cover off suggest that no other internal covers need to be removed. It does suggest that the spring goes on a lever operated by the trip lever or a lever moved once the shutter has been tripped.
Am I correct you removed the cover, cocked and tripped the shutter, the spring popped off, you made the picture?