I ran into this a number of years ago. There are two sizes depending on vin.

Start with the longer one. It’s easier to put on. Tension it up. If it doesn’t work get the shorter one. I did this with cheap Autozone belts. I went with the longer.

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79 300TD “Old Smokey” AKA “The Mistake” (SOLD)
82 240D stick shift 335k miles (SOLD)
82 300SD 300k miles
85 300D Turbodiesel 170k miles
97 C280 147k miles