Unfortunately, I have seen this. The older these engines get, the more I see in this condition. The rails become brittle after ageing. The only fix is to replace the rails and chain at 90k mile intervals and NEVER miss an oil change.
Mercedes V-8's are known for this in the 80's and now here come the 90's engines.
ASE CERTIFIED MASTER AUTO TECHNICIAN
MERCEDES SPECIALIST 11 YRS