How to service a Shimano TZ rear mech? I'm trying to swap out the top jockey wheel on a rear shifter mechanism.

The bottom jockey wheel simply unbolted as expected but the top one seems to share a shaft with the main spring rather than having its own bolt.

I can twist one of the cage plates a complete turn but it doesn't unthread anything.  

I can see a steel nubbin through a small slot, but its not shaped like a bolt head.

Is this Unserviceable?
These units are simply not meant to be serviced. The end of the bolt looks stamped like a rivet anyhow.

It may be a bit short as an answer but it's the (bitter) reality.