I didn't even know a Duralast belt was available for an MB. What do the markings say?

In any event you shouldn't be surprised. MBs go through the belts quickly anyway. If you don't buy the Contis you may expect early failure.

I wasn't aware there was any alternative other than Continental for MB belts, perhaps that's why I've never had a failure.

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'02 BMW 325i
'85 300D 450k
'93 190E 2.6 170k(killed by tree)
'08 Ducati Hypermotard 1100S 6k
'06 Ducati S2R800 14k(sold)