it looks like a retainer holds onto the end of that shaft. looks like an e-clip slot

 

It's actually a flat saddle key on the shaft and the encoder in the Throttle Body Cover. There is no undercut as you would have with an external ring clip.



I have no idea how it became jammed on the shaft, but that small rotational offset may have been part of the reason I was having this issue in the first place.

 

Indeed, the TB cover(tps)is the source of failure! Did you open the cover?

 

I didn't... but I will now! Stay tuned.