My best uneducated guess (617 experience only - if 606 is non-turbo then this is all BS) is that it's the ALDA. The black cap on the top is there to prevent unauthorized adjustment. There should be a large nut on the bottom of it that is captive on the IP. Turn it clockwise while holding the ALDA stationary (CW, that is correct) to release the ALDA.

On my ALDA, I cut the black cap off with a Dremel tool, then backed the nut off completely holding the center screw with a straight blade screwdriver, removed the remains of the cap, then replaced the nut. Makes a nice clean looking ALDA. If you are rebuilding your engine, then you will probably want to adjust this item anyway, so you might as well take it off and cut it while it's off the IP.

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mjk

'84 300SD 119KMi (Liesl der Diesel)
'84 300D 326KMi when the oil left (former parts car)
'82 300SD 253KMi (new parts car)