timing chain r&r vs woodruf key?

[r.e.wing_fc3s]

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.