Steve,

The big nut is for the rear flange. The only thing holding the cover is a big snap ring. Remove the snap ring by using something flat like a small screwdriver insert it into that small cut on the bottom of round cover. Once you remove the snap ring the cover should come out easily by sticking a bigger screwdriver into the slot that you mentioned. You can turn a bit and pull out. There is no spring inside that you need to worry about. Once you change the big O-ring and place the cover, make sure that one end of the snap ring overlaps the cut for future removal.

Vinamg