Is the large hole also angled? If so it sounds to me to be one of the drain outlets that fits into the sills. From memory there are four on W124s, W201s, etc. These things seem to come off easily like each of the four jacking pads at each end of each sill. It is not uncommon to see one or more jacking pads or drain grommets missing.

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107.023: 350SLC, 3-speed auto, icon gold, parchment MBtex (sold 2012 after 29 years ownership).
107.026: 500SLC, 4-speed auto, thistle green, green velour.
124.090: 300TE, 4-speed auto, arctic white, cream-beige MBtex.
201.028: 190E 2.3 Sportline, 5-speed manual, arctic white, blue leather.
201.028: 190E 2.3, 4-speed auto, blue-black, grey MBtex.
201.034: 190E 2.3-16, 5-speed manual, blue-black, black leather.