Isn't that the cover for the adjusting nut on the turbo encabulator?
I thought it was the anti-magnetic dissonance cap for the oscillating refrembulator.
Operation without this critical piece of equipment will result in the sudden and
catastrophic loss of turn signal fluid.....
(No, I don't have clue, either!

)
It does look like a cap for a nut or bolt, though....
-Stretch