The pin merely serves as evidence of ‘tampering’ for warranty purposes. It’s not necessary to properly retain the cap.
Its MB part number is 140 991 00 55. The O-ring is 022 997 22 48.

I’ve several in the spares bin if you need ‘em.

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Establishing the proper ATF level is imperative, as our 722.6xx transmissions are persnickety.
Use only 001 989 21 03 10 fluid.

MB’s #140 15 21 fluid level measuring tool is unlike a conventional dipstick in that it bottoms against the inside of the pan, and is then to be read up from there.

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Its markings at 25 degrees Celsius ATF temperature range between 10.3mm to 21.3mm.
Its markings at 80 degrees Celsius ATF temperature range between 54mm to 65mm.