That's likely the hinge pin that the hop up lever pivots on. I'll see what I can find with a manual search.
EDIT: Yup, there is plastic C-clip that is on the underside of the hop up unit, use a small slotted screwdriver to pop it out from the side, but be careful, those things break and will cause the above problems, and you'd have to buy a new hop up unit just to replace the clip.
http://www.depot53support.com/pdf/SIG552%20TM.pdf