timing chain r&r vs woodruf key?
 

though ive never checked i am assumiing the timing chain on my girls 85 300d 315k om 617.952 is longer then it was when new. mercedes make some sort of allowance for this and make available different keys? to account for this stretch. is there a factory change interval on the chains? otherwise i assume they wouldnt make something to accommodate a stretched chain if it wasnt acceptable to keep using it.

There is no factory recommended interval for replacing the chain.

To determine the health of a chain you must measure the valve lift over a certain range of crank motion. We call this the "2mm valve lift method". Do a search for this term or get a factory service manual (fsm).

MB does make offset keys to take up some slack in the timing chain on your engine.

you need to check the timing stretch to know how much you need, but if you are the original owner of the car and it's at 315K odds are good, you need new chain, new gears, and a retime of the IP.
there are pics online but you can get a close estimate of stretch by pulling the valve cover, and PERFECTLY lining up the timing marks on the cam gear to the cam tower. it will look similar to this -= if aligned the arrow has to EXACTLY center in between the hash marks.
note this is NOT an approved method of checking the chain stretch, it's just going to give you an estimate of stretch. the approved method is to use a dial indicator and remove lash in the valve... then lift 2mm, and note the timing mark, then use a chart to show what your stretch is...
anyway, 315K is likely too much...