Larry, do you hang out at the local Mercedes shop often? My Father's 420 threw the timing chain a few years ago. And when the S430 approaches 50K, we will change that one, too.
Last year I went by the local shop here in town and there was a 420 with heads off because chain was thrown. Last week as I went by I noticed the 190 four banger in the shop had a chain thrown causing extensive damage. Well, I wonder if changing an older chain in M/B's would be a good idea? Duh!! Go to a mercedes salvage yard and ask the guys about timing chains. The problem is easy to understand if you consider it takes a 5 or 6 foot chains to go around the bottom end up to one head back down into the timing cover and over to the other head.
For that matter, the short timing chain in chevy V8's may need replacing in as little as 50K. That is why my LT4 vette has a severe duty roller chain.