I have only seen them on MB cars with manual transmissions, the one you've describe and the one pictured in the EPC that I mention is the type that are installed on the 1986 300E 5 speed manual transmission cars that I have. There is a similar type ring only heavier/beefier that I have seen installed on 123 5 speed GETRAG manual transmission cars with OM615/OM617/M110 engines.

It may have been factory for your year car or perhaps the driveshaft was replaced. The ring itself is a seperate part most likely only available from MB I'd bet.