It has to do with pulley alignment, so only one will fit. Some engines have different crank pulleys and options. It might also be the difference between a naturally aspirated and a turbocharged engine and different vehicles and years.

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1967 W110 Universal Wagon, Euro, Turbo Diesel, Tail Fins, 4 Speed Manual Column Shift, A/C
1980 W116 300SD Turbo Diesel, DB479 Walnut Brown, Sunroof, Highly Optioned, 350,000+ Miles