I am not sure how many models had this problem. I have fixed them on 114/115 bodies only. I presume that proper tightening of the bolts could eliminate the start of the problem but my impression is that the whole area was just too demensionally instable to not fatigue over time (eventually is a long time).
As to the factory brace. It was installed on 116 chassis cars. I even did one in 1974 or 75 while I worked at the dealer (it wasn't done to fix a problem but to avoid one - maybe a recall or something). I also have one that I have recovered from a scrapped 116 car if anyone ever realy wants to see one. It is a plate (atleast 1/4in thick with three 1/4 in walled tubes welded to the backside. Maybe I'll take a picture later.
Bosch Master, ASE Master, L1
33 years MB technician