Not sure on m104 engines.

For the m103:

The slot in question is on the left (driver's) side just behind the pulley. It is in the 'casting' that supports the shaft. It is partially obscured by a metal coolant tube. The pointed end of the tool points forward toward the fan, and slips into a hole when the fan clutch is spun to the proper orientation. The tool keeps the fan clutch from rotating, and the slot keeps the tool from rotating. It is not much of a slot, really, more of a little depression. It may be caked in oil. You can hit it with some brake cleaner.

Best of luck.

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1986 300E 5-Speed 240k mi.