I know what Bud meant, but the 5mm hex key bolt holds the ROTOR on.
This job is pretty straight-forward.
I would suggest cleaning the sliding surfaces (surfaces where the metal pad backing slides in and out on the caliper) on the caliper with sandpaper, and applying a very light coating of thick moly grease (often marketed as 'brake grease').
It may also be worth checking the pins that the caliper floats on to see if they need grease. Mine were dirty last time, and binding a little. I use silicone grease here. Not sure what MB recommends.
1986 300E 5-Speed 240k mi.