It is a clamp holding spare tire.
Initially PN A 120 403 00 02
Used in: W105, W120, W121, W128, W180 - Ponton family in essence.
190SL on the picture.
Then, very similar clamp, PN A108 898 00 32
Went to: R107, W108, W109, W110, W111, W113 (here we are), W114, W115, W116 - pretty much everywhere...
It is difficult to find as it was replaced by PN A 110 403 00 02 for W110, W111 and W113 (here we are again).
It was used in Pagodas: Up to ident. no.: 042 007204 EXCEPT FOR: 006069 ALSO INSTALLED ON 007216, 007232. This is when vertical position of the spare tire became horizontal, I think.
Also found a picture with Pagoda trunk.