So I'm looking at the belt for the Fan/Alternator and it shows two sizes, although they are quite close.

GATES Part # 9395 HC50 15/32" x 39 7/8"}
W/39 3/4" Belt Outside Circumference; Fan and Alternator

or

GATES Part # 9405 HC50 15/32" x 40 7/8"}
W/40 3/4" Belt Outside Circumference; Fan and Alternator

This is for an '83 240D. If I had the vehicle here, I would check, but my wife is driving it around. Anyone with some insight? Its only 1" but I'd appreciate it.