Originally Posted by Graham

I wonder if some owners/shops stripped threads and then drilled out aluminum casting and put on a locking nut? If I can get the 100mm bolts to thread through enough, I could add a locknut (assuming it will clear the belts)

Looking at this epc diagram, it seems there are/were two versions. One with nut (Part 155) and one without (part 158). I think mine would have been 158 because I can get 10mm bolt in rear end, but not in from front. The part 155 and corresponding nut does not show down below in this link for 123.133 But does for 123.130 but even then seems to have been superceded.
https://nemigaparts.com/cat_spares/epc/mercedes/1/m/617952/67k/15/105/?chasis=12313368K



The location of that bolt is such that it has permanent rustproofing/lubrication - at least on my car No wonder it fell out!