IIRC stock is a 10mm hex nut with a locking nylon insert.

Many substitute the metal wingnut found on the power steering reservoir.
(MB part number 0009900962).

If you use a wingnut or a regular nut, also use a wave washer to prevent the nut from backing out.
(Just like the setup used to secure the air cleaner housing to the rubber buffers on the air cleaner bracket).

On the power steering reservoir you don't need the wave washer because the spring under the reservoir lid keeps tension on the wingnut (like a wave washer does).

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78 W116 300SD 'Desert Rose' new as of 01/26/2014
79 W116 300SD 'Stormcloud' RIP 04/11/2022