at all. The key factor with the chain rails is at least as much time as mileage; they're plastic and age slowly deteriorates them, making them brittle and increasing the risk of breakage. I seem to recall hearing the left inner guide is the most common offender, but it really doesn't matter. In your case, you'd have to assume the others are as old as the one that had already broken; they could well be the original parts. Remember that if MB recommended change at 100,000 miles, they hardly expected that to take 29 years to happen! My 560SL got new chain, tensioner, sprockets and guides this year at 148K purely for prevention, as I strongly suspected it had never been done. They found all the guides looking old enough to indeed be original--dark and brittle. One guide was half-broken and another snapped as they started to remove it--both being disasters ready to happen (and yes, I know and trust the shop).