The rivet is also the pivot for the cover. Pry gently upward at the base of the cover opposite the arm, it should flip up and expose a nut.
To check if the nut is loose try moving the arm in an arc. If the shaft at the base of the arm does not move, you have a loose nut.
If it does move, the linkage is broken.

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Usta haves '69 250/8, '76 280C, 1971 250C 114.023, 1976 450SEL 116.033
Current have, 1983 300SD 126.120